I got started with robots. I was in the antique district of our small town and one of the shops had a cart with 4 robot arms on it. The owner's son acquired them from his school. 1 of them was really old and in bad repair. Another was not terrible, but had broken cables. The other two worked just fine. I bought them all for $150. I ended up selling one of them, a Scorbot ER-3 for $600 (mine had the controller/power supply which most of the ones on ebay do not include). I sold a working Teachmover robot for $150. A broken one for $100, and the other broken robot for around $50. So, $900 for $150! The Scorbot actually went to India - the buyer arranged shipping via his DHL account.
After that score, I wanted to do more buying and selling. Auctions are ok sometimes, but garage sales are where you can find your biggest profit margins. That's how I got my start.