When buying a used rototiller, make sure to first start the engine. This allows you to see if it starts easily. If it doesn't, don't take it. Then, check the spark plugs. If you find carbon deposits or dark dust, it's a sign that the rototiller needs a tune-up. If the plugs are oil-fouled, it's a big warning sign to walk away.
Check the crankcase oil. If the crankcase is nearly empty, or if it contains dirty oil, then the rototiller is most probably poorly maintained. Other signs that reveal neglect of the equipment include cooling fins that are clogged, an air cleaner that is dirty and a throttle linkage that is either gummed with oil or rusty. These signs don't necessarily mean you shouldn't get the equipment, but you can use them as signs that future repairs might be in order.
Additionally, make sure to check if there are any oil leaks, if the transmission oil is low, if the rototiller has tines that are worn or bent or if you can hear any strange grinding noises. Finding a good used rototiller can be hard, but until you find one that works great for you, you can rent one from a local rental shop.