I tried to buy tractor and then resell them. Like everyone has said you have to buy or get it as low as you can and then sell high but often by the time you get everything fixed it put's the price to high.
Have you thought about maybe fixing them for folks instead of buying them? This may sound crazy but you can make some quick cash fixing trimmers. I would say 90% of the time you just need to replace the fuel lines and filter. Maybe an hr's worth of work $30 and the cost for the line and filter so about $37 and the customer is happy and you just made $30. If you do 3 or 4 of them in a day pretty nice. My problem is I just turn around and buy more tools or a new accessory for the bike. So I never seem to have any $$$.