if i buy off e-bay i buy new radio equipment and so forth fom the shops in china and so on, there prices are cheap but it is quite good quality most of the time, make sure you buy new stuff from ebay shops as they offer a return service.
as for buying engines and aircraft second hand, if your looking at an aircraft on e-bay and its fairly close to where you are ask them if it is possible to see it in person and to have a look round it, if it is a genuine sale with no problems or faults to hide they will be only too pleased to let you visit it and check it out, as for engines it may sound daft but send them a message asking if you can see and hear the engine running and if they replyt yes and offer you to go through you know it should be a good engine and not a faulty one, so which you reply il take ya word for it, its the fact that the person selling is willing to show you it working is what is important, as anyone selling an engine on there thats faulty or plays up and dosnt run correctly will most likely say no i havent got time sorry.
there are all sorts of tips and tricks to do and use to find out infomation about the item your bidding on by just interigating them without them knowing,