This is something that the father of one of my closest friends does for a living. He simply checks out dumpsters and roadside curbs during peak periods when students move out of dormitories at the nearby university, for example and scavenges anything of value, then sells that stuff at his own yard sale.
I even do this to a certain extent. I've been known to go to thrift stores looking for items that I can accurately price and then resell on eBay or at a yard sale. Occasionally, I hit gold I once bought a board game for $3 that I resold for $120.
You can easily do this, too. Look in places where there is little financial risk dumpsters after a moving day, thrift shops, and so on and try to identify items that would have value if brought before the right person. Then, take the effort to put them in front of the right person, either through eBay or your own yard sale.
There's not much financial investment at all. If you find someone who wants the item, you can make money. If you can't, then just get rid of the item again it's not a big investment, after all.
