I'm no pensioner and I do this. It's called flipping bikes. But I never buy them, I just pick them up off the street before the junkman does, I always ask the owner If I can have it first. If it's trashed and I find it in a neutral place, I just take it, get it working and get it sold.
It's a nice extra buck here and there. Sometimes, I'll just dismantle the whole thing and cannibalise it for ball bearings and other parts and dump the rest if it's too far gone.
Kret