I agree with the other answers with regard to LBS. Many of them are doing their best to provide bicycle services and aren't making a mint doing it.
It isn't a filthy-rich business and it doesn't make sense for most of them to sell used bikes. If THAT's what you'r looking for...
If you are looking at a bike from a garage sale, Ebay or Craigslist seller, be sure to look at the components and not just the frame. The frame, saddle, and shiny bits will certainly catch your eye first. Shiny doesn't necessarily make value.
The ANSWER to your question is to KNOW YOUR PRODUCT. Research bikes and figure out what it is you want and know what the fair market value is. Many private sellers have only a vague notion of what their bike is worth and may ask some percentage of what they paid for it twenty years ago. The bike may or may not be worth that now.
If it's a Schwinn Paramount, it's probably worth somewhat more.
Get to know components groups and what levels of quality they represent. Sheldon Brown's website is a great resource to browse before going shopping for 80's road bikes. If you know 600 and Ultegra are the same thing in terms of quality (and good) vs Shimano 2300 which is worth its weight in guano... you can start to get an idea what you're looking at.
What was the quality of the components new? What is their condition now? Will you have to replace the derailleur just to make the bike shift through its whole gear range? This is an added cost for a buyer. Are the brand-new tires covering up some ancient no-name steel rims that are going to cost you $200 to replace? That's something to consider.
Figure out what the bike you want is worth. NOT what you WANT to pay. What is it WORTH reasonably in the condition you want with the components you want...
It's not cool to lowball people or try to weasel them out of their bike. That's a couple steps above theft. It IS cool to know bikes and know what you're dealing with.