The best deal audio deal was a Yamaha MX-600 amp for $250 that was in-box, sealed, never opened. This was maybe six years ago and I must have caught it the instant listed. I still use it to run a second pair of speakers on the older system.
At the time, $250 or so was likely the going price for these, although not for NOS which is beyond unusual to find.
The best overall deal was 8 sets of original color toner cartridges for a huge Dell color laser printer. Paid $300 for all and they sell at $400 for a single set of four.
I've found a ton of really good bargains on eBay although these two are at the top.