I actually like this show and disagree with most the comments on here.
They don't force anybody to sell a thing. They ask to see what people have that they would be willing to get rid of for a buck. They offer a fair price that is below market value (think yard saleing) and they try to make a profit off of it.
In some episodes, I have seen them go back to the original owner and give them a cut of the profits. But then, I am sure that the original owner would feel cheated if they didn't.
It gives people an idea of what their junk is really worth. Most people have no clue.
A lot of the time, the stuff they buy the owner had no clue he had it, doesn't care about it in the first place or it will just rot away to the elements being uncared for.
Occasionally, not very often, they offer more than what the owner thought something was worth.
The owners usually contact them to come out and buy stuff, the driving around and looking on the back roads is a minor part in it. That was how they got their start, but now people are trying to downsize and get rid of their stuff. Frank happened to drive to my parent's farm and stopped to look around. He bought a few things (wish he would have bought more) and I got an autographed picture out of it. If you're reading this Frank, thank you...
Family heirlooms? People don't sell their family heirlooms. Never seen it happen on this show. Mostly junk there.

Frank and Mike work hard at what they do, they risk buying and selling (much like the stock market or gambling) and they make an honest living out of it.