How old is the fix? If the machine has been running a year or so like that then apparently it's a good fix & perhaps you can get a fair bit of time out of time out of it before a proper rebuild. On the other hand if the guy just did it a few months back and hasn't run it except to demonstrate it then I'd factor an engine rebuild into your offering price.
Around here a good running TO-30 with reasonable rubber and reasonable cosmetics is around $ 2500 - 3200. If someone offered $3500 for my TO-30 I'd probably sell her.
Just my $ 0.02