What is with the discrepancy on book prices on rare/used book sites?

[LuckyLib]

I have a number of books (30+ years old) and I am curious about their value. Old reference books from grad school, for example. On these sites there will be some sellers pricing them at $150, others $20 -- seemingly same type, similar condition, etc. If there are 30 books available, do they really think someone will pay the high price for the book?
I'm familiar with old/rare book designations regarding first edition, condition, etc. -- when that is not in play, what's with the range?

[NRaleighLiberal]

1. I think collectibles are made available by a whole range of people; some to make as much money as

possible, others to share treasures at a more realistic price. As with everything else, it seems, it is about what each of us is willing to pay....which varies widely. I collect old seed catalogs - at first, on ebay, you could get great ones at great prices - now flippers are involved that have no interest at all in the seed catalog - just as a way to buy low and sell high.