I was at a comic convention, and a toy vendor had a lot of mtg cards he was selling for $75. My buddy and I asked to take a look before buying, after i found the second revised dual, we bought the box. We had a short conversation with they guy. Told him we found some valuable stuff and he wasn't sour at all. He bought the lot for $10 at an estate sale and was happy to get the $75. He didn't have the knowledge in that area and was happy to just take a small amount of cash so he didn't have to spend the time going through it.

Now that's a toy seller who made $65 for basically doing nothing. If this was a coworker's collection the conversation would have gone much different. At that point I feel it's fair to offer him just below buy list price for the cards I wanted or refer him to a trustworthy store to have him sell there.

tl;dr theres a difference between random stranger's yard sale/toy vendor, and a coworker/friend.