Have you ever sat down and attempted to evaluate the value of your collection? There are a number of options to assist with determining values such as price guides and auctions. I know that some people will sit down, grab a Beckett and begin to count off all of their base cards then start to calculate the price of the semistars times the total number of base cards. This is how you get collectors claiming that they have a $15,000 or $20,000 collection when all they have are cards from the 90's onward. I have been collecting for almost 30 years now and I seriously doubt my collection even tops $10,000 but then I set reasonable guidelines for myself when considering the values.
Unfortunately not all sellers are willing to take any advice when evaluating what they have but they are the ones that are left in the end still holding the bag because they over inflated the prices.
Base cards, semi-stars and even some low-end short prints, relics and autographs are valueless. Just because it contains a jersey or is a 1/1 does not mean that the value skyrockets.