I have done well both buying and selling on gb. Keys as mentioned are to have a price in mind and not go over. I agree with regional issues like the Alaska poster mentioned. Before I moved to Arizona I was a prisoner of New York - horrible taxes gun laws etc. but some of the politics kept handgun prices lower than free states, and the shops I frequented always had dozens of used S&W revolvers on hand. Last year before moving I asked the shop owner for a shooter grade model 15. He brought out a bankers box with about 12 piled in there and told me your pick 300 bucks including tax. I ended up with a pre model number with matching stocks about 80%.
Since moving to free America I have stopped in many shops and have not found a single one with more than about six used classic smiths in stock and as otherwise mentioned shooter grade guns at collector grade prices.
I had been looking for a good fairly priced shooter grade 34 2" for the last four years or so and finically one a gun broker bid on one. By the time I paid shipping and transfer it was $550 which seemed high to me but ultimately I thoroughly enjoy the gun and probably over paid little. In all the shops and shows here in Arizona I have only seen one priced at $750.
When it comes to selling too, you get best money with a national audience. Some guns are simply far more common in some regions than others