I regged a dropped .net domain, gets about 300 uniques / month. Recently I got an email from the previous owner (i checked, he probably was the previous owner) - the email was very curt, to the tune of "....net is the intellectual property of mr smith you must immediately return this domain to its rightful owner...." etc.
I did some research, found that he did seem to be the previous owner, and since he was a fellow webmaster, out of solidarity I offered him the domain for a nominal $100 price to cover for my acquisition, transfer and other costs.
I never heard back from him, go figure.
Finders keepers, sure - sell him the domain at whatever price you will, it's your right.
I personally believe that business is not just about buying as cheap as possible and selling as high as possible. I believe business is about providing a service, and getting a generous payment for a job well done. That's why I am usually willing to listen to the other party in these cases. If they verifiably were the previous owner, dropped the domain in error, or especially due to personal crisis or health problems etc., I would consider that and lower my asking price accordingly, especially if it's not a clearly premium domain and the potential losses for me would be modest.
Just some thoughts. Of course, there is always the possibility of a scam..
Cheers
Josh