Unless it was the deal of the century, I personally would never buy a high-end fish from a LFS. The mark-up is just too high for me to live with, knowing that I could find a comparable/higher quality fish for cheaper online. This is my opinion, and I know some will disagree.
Regarding sitting on the bottom, some aro's do this. I would look for other signs of poor health (i.e. gill curl, thin frame, damaged fins, poor appetite, etc). It's also perfectly acceptable for you to ask an employee to see the aro eat. If you're spending hundreds of dollars on a fish, you deserve the piece of mind of knowing your fish has a clean bill of health.