I am not an advocate of tearing engines apart without cause, but I suggest removing the carb, backplate, and head BEFORE you try to run it. Check for dirt or signs of rust. Turn the engine over and if it's not absolutely smooth, it needs new bearings. Look at the liner to see if there are any signs of peeling. If there are, you may need a new piston/sleeve. Once you know the condition of the engine, look at the expense of fixing any problems and decide if it's worth keeping the engine or returning it (if you can).

I agree that it's risky buying second hand engines from eBay. I've had a couple problems (one that was supposed to be new and wasn't, another that was supposed to be almost new and had rusty bearings) and have decided that I will only buy engines that are guaranteed to be new.