Went along on Sunday. Bought a rare set of cards I thought I'd been chasing at a good deal as one card was missing. Bloke started doing the math in his head as to how much it was. Realised he'd underquoted by $20, told him as such, he was grateful for my honesty and we went on our separate ways.
Got to the car, put all my purchases through my collection database only to realise I'd already bought these ages ago. Considered going back in and seeing if he wanted them back. No, my issue to deal with, my mistake.
Went home, realised the one card I had a double for was the missing card from set I'd just bought.
Put it up on eBay for auction. Sold for double what I paid for it.
Not entirely sure that's the universe rewarding me for honesty, but a nice outcome just the same.