I buy most of mine used, but I never walk out of the store(s) without checking them out, and never bid/buy anything on eBay, Half, or Amazon unless they are super specific on the movies condiitons.
Hollywood Video, I open the cases, and break the plastic. Naughty me, I just like getting as perfect discs as possible.
Wal Mart dump bins are FANTASTIC places to find really decent flicks. Just today, I saw "Wall Street", "Casulaties of War" and "Matrix Revisited" for $5.50 brand new. I hit up their $3.88 sale this past Friday and got "Goodfellas" as well as 4 others.
For absolute new stuff, you usually can't beat DeepDiscountDVD.com. Otherwise, the best thing to do with new releases is buy it the week its released for the absolute lowest price.
I've bought 75 movies in the past 60 days, and I've only been able to do that by gettin' em used.