My father collected used modern British postage stamps, which is a very specialist field. For each stamp that was issued, he'd have dozens of different versions, each having slightly different colour tinges, variations in the phosphor bands, microscopic printing flaws, etc. His collection ran to many albums and, according to Stanley Gibbon's catalogue values, was worth tens of thousands of pounds. Both he and I knew that SG values are often around ten times what stamps will sell for between collectors but, even so, his collection appeared to be worth several hundred pounds and possibly even into four figures.
When my father passed away, I lugged all his albums down to Stanley Gibbons on the Strand in London. I was hoping that they'd make me an offer for them but, even if they didn't, I knew that they accepted entries for the auctions that they hold periodically. So I felt sure that they'd take them one way or the other. However they wouldn't buy them from me and refused to accept them for auction, as they said that it wasn't worth their while offering them for sale. I ended up taking them to a lesser-known dealer, on the opposite side of the Strand, and accepting just £50 for them :(