What to look for on buying used/open box systems from ebay for hacintosh?

Was looking at

https://www.ebay.com/itm/144047868366?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

Wondering what I should be looking for if I try to save money by buying used hardware configurations that would work with OS X.