Sale prices for used bikes and parts are largely fixed. When flipping, you make your money on the buying side. You have to pay as little as possible for bikes. Buying prices depend on the bike’s condition, the seller’s financial state, his current mood, and other fuzzy factors. Your best bet at increasing your margins is to make an offer just north of “insulting.” That means unrepentantly beating up the seller. So, if the seller doesn’t have a pained expression after your offer, you probably left some money on the table. Most people don’t have the heart or negotiating skills for this part, which is why there are so few pro flippers.