I had a 5x8 single axle dump for about a year, then went up in size to a 6x10 tandem 10k. Big difference in amount of stuff you can haul, obviously. Glad I did. Would have gone bigger, say 12' but I have a tight storage area. The 10' still turns pretty sharp like the 5x8 did, so not much difference there. Pulling it with both f250 and the Chevy 1500, which does a great job with about 4k payload.
Bought 6x10 new because used dumps don't loose much value. Bought 5x8 used and sold it for the same amount a year later. Leaky hydraulics is about the only thing I'm aware of to look for. Just be aware that if you need to install new tires and a new battery, that will add serious $$ to the purchase price of a used one.