the only steps you can take to protect yourself when buying a s\hand performance engine is to hear it run and to do a compression test on the engine.If you are not familiar or not comfortable doing stuff like this it would be best if you shout someone a carton of beer to enlist their help.
To hear an engine run you will either be purchasing a front cut at 2500 to 3800 depending on manual or auto etc OR a motor in a persons car that is smashed [good way to go] or that is to be swapped out for another reason [suss?]
Alternatively you can buy an unknown engine without doing any tests and get it for as little as $500.I bid to 490 on an ebay 1J recently only to see it sell for 495.Spewing!
You can buy them for 700 at a wrecker with no turbos [the ebay motor had no turbos] best to use the search function on this site,try and become familiar with people from near you that post a lot with the knowledge you need and ask them for assistance if you need it.Good luck.Pete.