It's one thing if a bike is "a little" on the big side. Depending on leg and torso length, adjustments can be made to frames that lie on the farther ends of the scale.

Selling a bike to a newbie that is clearly out of the ballpark is quite another. Any experienced bike seller/fitter/rider would know that it was too big. Trust me, your salesperson knew this. If you bought an '09 bike in late 2010, he was simply trying to get it off his floor, at your expense.

(BTW, you mentioned aerobars. If you are using the bike for tris, your frame could even be a tad smaller than a road set up)