More so in the early days of the show, but I think that when it is a legit call, the show shows what they think would make good T.V.
They can't keep showing Mike and Frank buying Texaco signs every episode.
I noticed that too, where they drive to a place for hours, then they only buy like 200 bucks worth of stuff, yet there are HUNDRES of signs etc there for the taking (when the seller says everything is for sale).
I'm sure when the cameras are off, they haul in everything that's worth haulin.