I ask: has the domain got links that I could not build or buy myself? In other words, are their backlinks from high authority websites, preferably with lots of trust too? If it has one/some of these then it's in my good books already.
Then it comes down to price. I crawl for my domains so get them all for reg fee but if you are buying them in auctions then you'll have to compare domains and work out which is best value for money as your ROI is the only important thing at the end of the day.