Start a new game with fellow NPC
Ask him for giving you some equipment
Take the gift
Trade with him
???????
Profit! No, really, see some strange "profit" sum for nothing.

Win 10, 0.C-4103, tiles.
Confirmed in 2ccfde9
And now that ancient song is stuck in my head. Thanks for that.
You can also break down a bench and sell the 2x4s for around a hundred bucks each.
@Chezzo
Each 2x4 itself is worth a hundred bucks each? That's insane!
This should only happen if the given item has negative price. Negative prices were fixed a while ago. Does this still happen? Any reliable way to replicate?
@Coolthulhu As for 0.C-4417 the bug still exist. Manual in first message of this issue is working.
Nowadays this bug has it the other way around. Trades after the gift will "cost" the value of the gift.
Discussion's been stalled here for a long while. Closing. And according to @Vidiveni's note, it might be worth opening a new issue.
Reopening, bug is still here, reproduced it a few days ago.
Most helpful comment
Reopening, bug is still here, reproduced it a few days ago.