Make sure that:
1. it is sealed from the manufacturer and has never been opened
2. legal, original and not copied
3. includes a unique and valid (never before used) registration key
4. it has no limitations - in other words, it is not some kind of a 3-month demo, or an unlimited-use demo with certain features turned off
5. seller has good reputation
6. check out the shipping price. Sometimes, they charge 50 dollars for the software and another 50 dollars for "shipping and handling" and that's where they get you.
7. Also, this sounds dumb, but don't get currencies mixed up. If you are in the USA and buy something from England, well.... 50 pounds are a heck of a lot more than 50 dollars.
If you are uneasy about it, you could post some links here to the items you want to bid on and people here in the forum can check it how legit it is.