Aside from the internet, there are also gun pricing guides in your bookstores or cheaply available through Amazon etc. Gun Traders Guide is one, The Bluebook of Gun Values is another, etc. Even a slightly older edition can still give relatively good pricing information in one place plus ways to identify the firearm and establish condition enough to determine if you are being lowballed in offers. Also, if there are a lot of firearms to go through, the pictures in the book can help narrow down what you have and give you ball park figures. It would be less tedious to do that then countless internet searches of images, etc. The internet is great if you know what you are looking for and what you are looking at--but there is a lot of misinformation out there like inflated prices, inflated condition examples, etc.
Where most gun value guides fall down is if you have something off the beaten track as firearms have been around for centuries by a large number of mfgs. and not all of those variants make it into the price manuals. The market in such items is also shallow which makes pricing an item more difficult. Specialist collectors, if they are aware of an oddball variation of a common firearm, might well pay far beyond a std. guide's pricing for that common firearm.
Easy example is a Smith and Wesson No. 15 revolver (1957+) with a five inch barrel. This would be a special order from S&W and would constitute something that a collector might pay more for than a std. 4 or 6 inch barrelled one which price is reflected in the std. manuals.