I do it all the time.
It's called karma.
About six years ago I stumbled on an auction for a Martin D-45 guitar in pristine condition, with a Buy It Now price of $500. The auction had just been posted and I asked the seller if that price was a typo (a Martin D-45 in that condition is worth more than $5,000). The seller said it was her son's, who had recently passed away, and she had no idea what it was worth. But she was so impressed and thankful that I contacted her, she said I could have it for $500.