A couple of things to look for when buying practice GMAT tests:
- Are the tests CAT tests? More specifically, will you be taking them on your computer or printing them out? I recommend CAT tests that you take on your computer.
- Do they include all sections for each test? (Two Essays, Quant Section, Verbal Section)
- Do they allow you to see what you missed at the end and give explanations of the right answers? Further, do they allow you to print the questions you've missed?
- Do they have a timer? This is crucial. 800Score.com has a Test Pacer that shows you the overall time as well as what question you should be on. This is VERY helpful for learning how to gauge where you are on the test and where you actually should be.
If you can answer all the questions above, I would recommend buying the practice tests for the GMAT.