I agree with Buz. Best to take bus or have someone drive you to this guys house and you ride it home.
If you don't want to do that, check out U-Haul they have Motorcycle trailers for rent in some locations. Don't put it in a pick up unless you have the proper gear. Or if you may want to invest in a trailer try these links. When my wife was being a ***** about dropping me off to get my bike for service I threatened to buy a trailer. She stopped.
I just trailered my bike to Murfreesboro, TN. that's about a 10 hour drive. I had a tie-down snap on the way. Here's my advice...
Use two tie-downs on each side, and a wheel chock. If you can, another tie down on the rear of the bike. Make sure you tighten them until the forks are compressed. Don't have to be all the way compressed mind you.
Put a cloth between the tie-downs an the bike if it looks like they make touch the fairings. No scratches!
Make sure you ain't crushing the brake and clutch lines with your tie-downs too. That's never good.
If you've never done this before, my advice is find someone who has, and take them with you. Buy them a 6-pack to make it worth their while.
I was considering the purchase of a 2004 Daytona 955i that was at a dealer 600 miles away. Not wanting to ride a new (unbroken-in) bike that distance I found a company called Shipping Masters that specializes in moving bikes.(http://www.shippingmasters.com/pallet.htm) They will give you multiple quotes from transporters who will deliver your motorcycle to you. My quotes ranged from $629 to $450 depending on how the bike was going to be transported (individually crated, palleted, etc.), and the pickup/dropoff location. I ended up buying a Triumph locally (about 60 miles away) so I can't attest to how well the system works, but it sounded great. Still, I have to agree with the other posts and say you should just ride the bike back if it really is only 100 miles away.
You can buy a strap that slips over the grips called Canyon Dancer that provides a good attachment point for front tie down straps. You need to be cautious not to strap the bike down too tightly when using this device because it can put undue pressure on the right side throttle grip.
I cut the ends off of old socks and slide them over the tie down straps placing them at points on the tie downs that might rub against the bike. You can secure them in place with tape. I would ask the owner if he will siphon all the gas out of the tank. You don't need all that weight sloshing around up high on the motorcycle. If the bike rolls up to any kind of front wheel chuck I would strap the front wheel to the chuck, protecting the wheel paint with soft cloth from strap abrasion. No one has mentioned this yet, and although it seems obvious, I have seen people do it so, don't strap the bike down on its center stand.
Due to a spousal health issue, I frequently trailer my steed...re advice on tying down I have nothing to add. Darn good advice here. Note that Canyon Dancer harnesses will eventually bugger up your grips if you trailer more than occasionally.
Since I have to trailer a lot, here is the bottom line - buy a BAXLEY trailer...period. There are plenty of ok to pretty good trailers out there, but get a BAXLEY...period.
You can load and unload by yourself. You can secure your bike WITHOUT crushing the fork springs - and shortening seal life. You can secure just about any bike - including those emtombed in plastic since you only tie from the rear of the bike.
Yup - expensive and does not address your current situation. Too dang bad you are way out on the left coast - those Rampmaster ramps I've got would be the "bees knees" - buy an LA Chock, borrow some ramps, get yourself 4 ties (NO rachets!!!!) and rent a pickup.
You don't have a friend who will drive you 100 miles? First, work on your personality a bit. Then, with your newly acquired social skills, try to talk the guy into delivering it. If he still won't, at least you stand the chance of going out, making a friend and getting him/her to drive you the 100 miles to get your bike. Be sure to say thank you to your driver, offer to take him/her out to dinner and quickly return the favor by helping that person out with some difficulty they are having. These skills will last you a lifetime and will make your stay on this earth more pleasant and fruitful.
Lots of good gouge above. My tuppence worth (and I'm not trying to insult your intelligence, I'm assuming you've never done this before.):
Find a guy with a longbed (8') pickup truck, or go rent one. A shortbed will work, but the tailgate won't clear.
Get a set of four to six soft-ties/cheaters/loops to make up to the bike.
Go to Sams or equal; they sell good quality ratcheting tiedowns. You'll need at least four, six is preferred.
You'll need a ramp of some sort. Wider is better (Sorry, I know Pontiac ruined that line.).
The 'Triple is a piece of cake to tie down, as it has the big bars, and the stout passenger footpeg brackets.
First, try to back the truck into a ditch, so the tailgate is closer to the ground. Rig the ramp so it can't move. Put the bike in the back of the truck. Put the bike in gear to prevent it rolling. Center the front and back wheels. Put the sidestand down, and leave it down until you unload the bike (You'll find out why when you trip out the front tiedowns (Throttle side first)).
When you ratchet down the bike, don't go crazy; just take the bike down evenly, starting with the clutch side, until you cannot push the bike down with your weight. Don't overcompress the forks. Doing so may damage them, and I've seen several truck beds where the front panel was bent forward because of overexuberant tiedowns.
Make up the rear tiedowns to the footpeg brackets. Tension them to the point where your weight does not compress the rear shock further.
Use the fifth and sixth tiedowns to immobilize the front wheel. I make up a cheater to the bottom of each fork, looping them around the axle. Take the other end to the cleat in either front corner of the bed (Should be the same one the handlebar tiedowns are made up to).
I have rigged my F-150 with stakehole cleats, where I can attach preventers, but I usually use them only when I'm hauling two bikes.
Lastly, use some wire/string/duct tape, etc. to tie up the tails from the tiedowns. You don't want all that strap flailing around, especially since they could get fouled under the back tires!
Do a good walkaround to ensure none of the straps are close to the bike's bodywork, and you are good to go!
However, the better answer is to ride the bike home.
Rent a U-haul garden trailer, or one of their open 4 X 8s. Buy four tie-down straps from U-haul and a couple of those black rubber straps with hooks. Don't buy bungee cords.
Have the seller help you push the bike up into the trailer - here is where you want the garden trailer or you will need a ramp.
While he holds the bike up you slip one of the strap hooks around the handlebar and then to the front rail of the garden trailer. Then go to the other side and repeat. Now he can let go and you can put the remaining two straps onto the back shocks, frame or swingarm and then to the back of the trailer.
Now you have four straps pulling in opposite directions.
Next, have the seller push or pull down on each shock while you cinch the strap. You want to compress the shocks as far as possible. This is important. Especially the front shocks
Finally use one black strap to tie the front wheel solidly to the front of the trailer. Use the second black strap to tie the rear wheel solidly to the floor of the trailer - if possible.
The bike should be cinched to the trailer so tight that they move as a single unit.
All this requires a vehicle with a trailer hitch. If you still need that then I suggest getting a bus ticket to the seller's hometown - I'm sure he'll pick you up at the station - then ride the bike home.