You also have to think if what you paid for it is worth it to you and would I get the same in return in reselling it. Most of us buy to collect and not sell, so any coin I buy for under value I am excited about. Like others have said, there are many places to look for a value:
RedBook, CoinValues, Greysheets, PCGS,...and the answers are almost a personal level. Is the coin graded, raw, silver, gold, variety, or common also play into the factor and all need to be considered, just because I say it is worth $30 does not mean you could not get $50 for it.
The first thing I try to do is find a dealer that I like and feel I can trust and see what they are using to set their prices. Most of the time it has been the
RedBook or CoinValues and I gauge my coins from there. The difficult part is that if you are trying to sell them, because then the Bluebook comes into play and the prices are completely different. The idea behind the Bluebook is a base price to buy coins from individuals and still be able to make a profit from them when reselling them. Just because I have a $30 coin does not mean I will get that for it, most likely I will get $7-10 for it.
ebay is a good and bad place to look for coins because there a large number of people trying to take advantage of inexperience and sell their items for a lot more than they are worth. What complicates this more is that most of these people do not know how to grade and mark them up a lot more than they really are. All of us have seen a coin marked as MS63 when it is barely a VG8 or the dealer that slabs his own and has perfect MS70 on his entire stock. Because of these issues it is hard to figure out what the real value of a coin is. The other trick they use is to reference their coins to the PCGS coin price list. The problem here is that the coins were never graded by PCGS and no where near the price or grade they show. My first purchase off of
ebay was for a set of four Lincoln Memorials that were priced at $12,000 on PCGS that I ended up buying for $10. The coins were nice and worth the $10 I spent but no where near the grade, condition or value PCGS puts on their coins.
Websites like this are a very helpful place for multiple reasons:
1. There are a wide range of collectors from novice to dealers with specialties in about everything in between. The information you find first will help you in figuring out grades and a price range better.
2. This site is set up by collectors for collectors and the reason for its existence. No one here wants to see some one cheated out of a coin and will help where they can. This is also shown in the buy, trade and sell areas where members post their items they want to sell and do it at a reasonable price. You can buy something from here cheaper than a dealer or
ebay a lot of times and by selling something you have helped another collector by giving them something for a cheaper price than normal.
3. Knowledge. There are a number of sites set up that focus on one type of coin with its varieties, errors and other useful information. By informing others they help with the collecting end and getting more people into it and looking for something out of the ordinary.
Now, I am not saying that everyone on
ebay is a crook or that dealers try to make a profit off of you, but it is a business. Like everything else you do, you should research it first. You do not just show up to a car lot and point out a car and say I will buy it without looking at it and taking it for a test run, and the same should be done with collecting. The more you know the better to figure out the true value of something.