Got a couple;
i) My original A500, which I managed to get from Boots for £200. My mum worked at a store at the time, and knew I really wanted one, and I had been washing my parents cars every weekend for a year, but was still way off the £400 the packaged A500 cost. Someone brought one back to Boots after Christmas claiming that it didn't work. The manager of the electrical section told my Mum who suggested that I could have a look at it, and see what was wrong with it, as he suspected that nothing was wrong with it all. Had a look and it was fine, just missing all the packaged games, although it still had Workbench and DPaint II. With all the missing games we managed to convince the Manager that I could pay £200 for it, as that was about the price Boots actually paid for them. Only condition was that I had to test all the games that they got returned as not working. Which turned out to be really good fun, as I got to play loads of games, as most of them worked fine.
ii) Second best deal was much later. I was at Uni and my girlfriend said that she wanted a Computer to finish her 3rd year project with. Was going to get an old Laptop, but she said she actually wanted an Amiga. Put an advert on Amibench and managed to find a guy in London willing to sell a A1200 with PPC card for £120. When we got there he also chucked in loads of other stuff, like a complete A600. I kept the PPC and put it in my Power Tower, and gave the A1200 and my old Blizzard '030 to her. Then about four years later I sold the A1200 for £50 stock, and the PPC card for over £140. So after four years usage I actually made money. Only with the Amiga
When downgrading from my PPC Power Tower to a CD32 and SX32 for size reasons, I advertisied on Amibench for a CD32. A guy got in touch pretty quickly claiming he had a CD32 which was brand new and still boxed. When it turned up there was no power supply, and even eventually after I bough another CD32 from ebay, the one he had sent would not power up. Guy claimed he had never said there was a power supply, and I had never asked. Cannot remember what it cost but was about £50, I think.