Denis, you are right about people making a living buying and reselling. You are basically getting paid for your knowledge. I happened across a couple of saxophones the other day at a yard sale. Bought them both for $75.
Just sold the pair yesterday for $322 for the alto and $815 for the tenor.
Tenor Sax
Alto sax
I didnt' know exactly how much they were worth, but I knew they were worth more than $75.
I don't feel bad about that deal because that guy was just cleaning out a closet and was glad to get rid of them. He actually wanted $50 a piece! But said he'd take $75 for the pair with no prompting from me.