No way you should leave this at "lesson learned"... you paid full retail price for a NEW computer. Go back to the store, show them the receipt (establishes Sell Date), show them the console logs (establishes Used Date). If you have to take the computer into the store to prove it, do so. I wouldn't leave that store again until I got:
1) If you want to keep the computer, a refund or adjustment equivalent to 20%, or their normal "open box" markdown, and make them foot the bill for extended AppleCare, or if you bought from Best Buy, the in-house (called "Black Tie"? not sure anymore) service/warranty plan
2) If you want a brand new computer, some sort of gift card (if you ever trust to shop there again) as a "We're sorry."
If the manager knew of this practice, I would also EECB (executive e-mail carpet bomb, a la the Consumerist) corporate and support channels.
If the manager didn't know of this practice, I'd expect that the salesman will get an earful and maybe even get fired... at least I would hope that, probably won't get to see it though.
Seriously, accept nothing less than a replacement with some sort of inducement for your troubles, or if you keep it, get a refund for the difference and some sort of warranty support thrown in.