I had someone do this to me with some Rollei Retro 100 film. He had some listed on eBay, a 10-pack in the wooden case, for like $35 BIN. I bought. He sent me a message saying he had more and did I want that? I said "Heck yes!", well actually I played it cooler than that. But he had 3 more boxes of it and offered them to me each at the same BIN price on the auction I won. I quickly paid for the rest with paypal. He then sends me a message the next day saying he spoke to a friend who told him that Rollei Retro 100 is actually Agfa APX100 and quite rare and worth much more so he told me was still going to send me the 10-pack I won in the auction (oh, are you? That's nice) but that the rest he was going to put on ebay again as an open auction. He soon then realized I'd already paid him and felt kinda bad. He refunded me and did send me a couple of empty Rollei wooden film boxes as an apology.
These people should have honored your $75 if you had an agreement to sell/buy at that price. Heck, even the law I think would be on your side but it's not like it would be worth it to fight them.
I got a 2.8E Planar Rolleiflex from a tiny shop down a dark alley in Beijing back in 1999 or so for $150 USD. A little beaten up but it has been to Harry Fleenor for a "spa treatment" and shoots like a dream ever since. My other Rolleiflex score was a 3.5E3 Xenotar for $120 USD. This one obviously had been dropped as the side knob was a little dented in. But it worked fine and still does!