Over the years there are some deals that I wish i had walked away from, some that I wish i had jumped on, and some that I'm glad I did walk. One of the things that helps when looking at buying some, is what's it worth to you. That may be an easy question to answer, but there are some factors to keep in mind.

What is your plan with this bike? Do you plan to keep it and have some fun, fix it up and sell for a profit, part it out, etc
What the market showing they are selling for? Look at ebay, craigslist, forums, etc.
Does it run? If not, have a fair idea of best care and worst case on parts to repair it.
Does it have a title? If not then keep in mind how much the bonded title process is in your state.
Do you have to ship it? If so, check uship for quotes and also make sure you completely understand the current condition of the motorcycle.

One you have your homework done, then make sure you have a top dollar in mind when going to look at a motorcycle. When you go, only take out that amount of money with you. Trust me it helps with rash decisions. When looking at a motorcycle here are the things I look at;

General Condition. I've seen some motorcycles that sat for 20 years that look great and just show age and layers of dust. Others look like someone thought they were Evel Knievel and they should never have owned a cut off wheel. What you are looking for is trying to understand the story of this bike. This is to make sure it's in line for what your plan is.
Frame, Forks, Wheels, Suspension. Check this for damage to see if it's been in a decent collision. Wheels don't worry me, but it's my indicator for where to look next. If a front wheel is bent, look at the forks to see if the legs are straight, inspect the frame neck and so forth.
Motor. Look it over well. If the motor is free, bring a compression tester and get the numbers. If the compression is out of spec, expect at least a top end rebuild. If the motor is locked, plan on worst case with full rebuild cost. Looks for broken fins, look for any damage to the cases, and any leaks.
Fuel system. Hopefully there isn't gas sitting in the bike, if it's been sitting. If there is a gas and it's been sitting over a year, look inside the tank. Gas can go bad and become a gel like substance that isn't easy to clean. Look and see if the petcock is off (hopefully the bad gas wasn't sitting in the carbs. If you can, pop a bowl off one of the carbs and look inside
Electrical. Look to see if all of the components are intact. Does it look like a mouse made a home in it or some just went crazy cutting wires.
Bodywork. Looks for pinholes, rust, creases, dents, missing pieces, missing tabs.
Paperwork. If there is a title, make sure the paperwork matches the vin on the neck. Some motorcycles did not have identical numbers on the vin and motor. I have see some older titles that had a number swapped on the title, or they used the motor number in place of the vin. If you encounter this, most states will make you go the bonded route. If there isn't a title, then do what you can to check that vin. Most local law enforcement agencies will run a vin if you ask them. There are some state websites that can also do that. Get a bill of sale and plan on doing a bonded title.
Ask questions. Find out what they know about it. How long have they owned it? When did it run last? If anything seems off, then make sure you double check it all yourself.

Alright, if you are still feeling good about this bike, then it's time to talk money. Try to gauge the seller through conversation and while you are there. See if they just want the space freed up and the bike gone, or they may be completely firm and will hold out for that exact number. I typically throw out a respectful offer that's slightly under what valve i have for it. Let them see what they come back with. If they counter, then evaluate where it's at. You may try to meet in the middle to close the deal. If they won't come in line with your top dollar you have (after you have looked it over and done your homework), then say sorry i can only go this amount.

There is no harm in walking away. It's better to waste an hour then eat money on a deal. I have found myself trying to convince myself, "These are hard to find, you won't find another one." Everytime I've thought that, i see one down the road. Hold out and find what fits for you and your goals.