you should be able to make it work. inseam and height are only part of the equation though. you'll see stack/reach is a very important bit, too. you might have to play with the stem and the height of the bars. hopefully he/she left you some to play with as far as the bar/stem height goes. as for the length, you might need to go shorter or longer depending on your angles.

with tri bikes the fit is a bit tricky. a lot of people like to go smaller, myself included. i would guess just by inseam/height and assuming you are a proportional person that you would be a 54-56 road bike so a 53 with a 53-54cm tt in theory would be usable.

i couldn't even guess if it was a good deal or not. i saw an older one with an ultegra group being sold for 7 - something at a shop today. so if yours is newer with da or red parts i'd say you did well.