Most engines sold on Ebay will be second user engines, which have been removed from cars or vans.
The service history is not often available, so the following precautions are what we would take.
Fit a new timing belt kit or timing chain.
If you can afford it, replace lifters and replace the head gasket or recon as much of the engine as you can afford, but never skip on the timing belt even if you was told that it was replaced XXXX amount of thousand miles ago, also try and buy the timing belt from a main dealer as cheaper belts may not last as long as expected & could cause your engine fail.
Replace the oil filter.
Engines are normally despatched without oil, so remember to put new oil in. If the engine is more than 5 years old, or has done high mileage, ask your dealer or mechanic, for advise about using thicker grade oil.
Use the correct coolant for the engine, this is important in the summer and the winter as it acts as anti freeze in the winter and cools the engine in the summer.
Make sure that the fault with your old engine was not caused by a fault on the vehicle, for example; an over heated engine may have been caused by a blocked radiator, leaking water pipe or it may have had a blocked or leaking oil cooler or faulty cooling fan or even faulty warning lights leading to the unnoticed over heating. The domino faults affect.
When ready to start the engine, crank the engine for about 10 seconds without starting it to build up the oil pressure, you can remove the fuel pump fuse to stop the engine from starting. Look out for fuel leaks. DO NOT take off the HT lead for this reason. Some vehicles may take a little while to start due to air locks in the fuel line or rail.
When starting the engine for the first time, let it tick-over at first & don't rev the engine to high straight away, unless new lifters have been installed, in which case hold the engine revs at around 3k for a couple of minutes, but please refer to a user manual.
Keep a good watch around the engine and under the car for leaks as the engine slowly reaches it operating temperature. Do not top up the water if the engine is hot only top up while the lid is off before it warms. The engine exhaust manifold may smoke a lot when it starts to warm up due to oil on the manifold which is heating up and starting to burn, oil normal ends up on the manifold because of oil hands touching it or spills. Also you may smell burning oil for a few days after the engine has been fitted as this oil slowly burns away, but if the smell does not go, then have it checked for oil leaking.
Make sure you are buy the right engine for the vehicle, it is always better to check if it will fit via a mechanic or you can get good advice on many car forums.
The best advice we can give you is, have the work carried out by a qualified mechanic and therefore, we do not accept ANY responsibility for any errors or mistakes due to the information or advice given herein.