Often, your bike won't be running. So you can't ride it home. Even if it runs great, it might not be the best idea to ride it very far before you can give it a good look over. When I go to buy a bike, I always bring with me the means to transport it. Usually that is in the form of my truck.
When I sell a bike, I usually offer transportation services for gas compensation. However, that also means that the asking price is now non-negotiable. People who have the foresight to bring a truck or a trailer and take the bike themselves usually get better prices from me. Often the discount they can get from me by haggling is MORE than they would have spent on renting a truck or a trailer. Lots of motorcycle sellers think this way, if they offer transportation at all. So keep that in mind.
If you don't have a truck, you can rent a trailer from U-haul for a day for pretty cheap. Usually these can be towed behind almost any vehicle. If you don't have a car or a truck, you can rent a U-haul pickup, or from Home Depot by the hour. You can borrow one. Hell, you can stuff the bike in the back of grandma's minivan if you have to. Point is, if you show up to a motorcycle haggling session with no clue on how you are going to get the bike home, you have put yourself in a very disadvantageous situation before even knocking on the seller's door. Do some planning, and figure this all out before you leap into the situation.