Ok, we need a new engine. It doesn't matter what size, ours has quit due to abuse, dirt or plain worn out. Some of the major brands and some of the older major brands can be rebuilt, but as in the case of O.S. engines, the price of parts is prohibitive and in the case of say Veco engines, it is very important to know where in the company ownership the parts were made and how that compares with your engine. When I am looking for a used engine, I look at it closely, it should be clean, turn freely, have compression and the carb should be rotating or sliding. I stay away from engines sales that say I haven't heard it run because I don't have the equipment to start it, unless it is only for parts and a parts price. I also stay away from engines that say it just needs a .... If I wanted to rebuild one I would rebuild the one I have. This is especially common in high end racing engines like Rossi, Mega, RB Concepts and the like. If a racer is selling an engine, it is going to cost nearly the price of new to make it like new, and he has just bought the latest model for a few dollars more. I look for ad's that the engine that came in the car is commonly upgraded to a larger engine. Traxxas and HPI are good for this as well as Associated, since all of them put smaller engines in their entry level cars hoping to sell you a larger engine. Usually these engines were swapped out fairly quickly or never run at all. Kyosho engines are commonly swapped without ever being started. Remember that 3 to 5 gallons of fuel is quite a bit of use on a small block engine and even a big block considering that 175 cc's of fuel will run it for 20 mins and there are 1000 cc's in a litre, it adds up quick. I also stay away from broken pull start engines because it indicates to me that the owner does not have enough experience to understand not to tug on his starter like a lawn mower engine, short quick strokes is the key. Broken ones are usually caused by over pulling the starter cord and can cost half the price of the engine to repair especially if the one way bearing is broken. I always ask questions and take nothing for granted. Ask if after run oil was used every time it has been run. Make sure the seller actually owned the engine and ran it, not found at a garage sale or bought in a parts lot. Ask them why they are selling it, if it is because he had problems, they will quickly become your problems, and yes, some are better at tuning than others, but still, it could be a can of worms you don't want to open. Last, check there feedback and ask them if you get it and it doesn't run if you can return it say within 3 days. If it is a good engine and the seller knows it, most won't object to this, if it is bad and he knows it, and refuses admantly, then I get leary. All and all it is like everything else used, it is a gamble, but armed with the right information, it can at least add to the odds on your side.