As a collector, who only sells duplicates I'd make the following comment records are listed at different prices because they are worth different amounts to the person selling them. I rarely list items here or on ebay but often have a small box out with me for sales. If I take the box out and don't sell anything I don't care particularly. If I price something at say £50 I may sell it at £40 if offered that for it or I may just prefer to keep it sometimes it's nicer to have multiple copies of things than a few more quid in the bank. I've sometimes sold records at the prevailing going rate and regreated it because in hindsight they were worth more to me than the going rate some times I've done the reverse. I sold a spare of E. Lampkins - Music Turn Me Loose on here in response to a want being posted, I've never seen one for sale, most people don't know it, it's not listed in any price guide I let it go for £50 I don't regret that even though I've been offered many times that for my other copy since because to me it was worth £50.
A record isn't just worth what someone will pay for it, it is also worth what the seller will not sell it below (at least to the seller).
Dealer's are different to them it's stock and there little point in having dead stock lying around tying up cash that can be invested in other stock that will turnover more quickly.