I used to enjoy going to stamp & coin shows back in high school, but I was never a seller, and I could never afford to buy the stuff that I really wanted. This was probably a good thing, since I had no skill at evaluating the condition of the stamps or coins. Now, I haven't been to a show in decades. I definitely remember that this is true, however:

And you're not going to get the "mint in original box" price for an item your grandkids have kicked around your yard in the dirt for six years! But some people simply don't get that... or they don't want to get it.

When estimating my own collection's value, any time I wasn't sure, I always wanted to assume that the stamp or coin was in the better of two (or three) possible conditions.