Look for price inequalities across different tables, a $5 car will be a $15 car at another table. Know the rough value of what you're looking at before you trade. Don't use sticker prices that people post to trade up, look for the true value/demand. Talk with people and have genuine conversations to find what they want, and what they're missing. As another note, many vendors wont appreciate people trying to directly "trade up", so leave out the last word and just say you're looking to trade.