Do you know if the tank is currently running, with fish in it?  If so, you'll save a lot of hassle if the tank is already cycled.  (look up 'cycle' in quicklinks to the right if you dont know what that is).  Getting the cycle going is the #1 cause of new fish tank disasters (all the fish dieing!).  If you can get yours jumpstarted, then all the better.

For smaller tanks, the tank itself is usually the cheapest part.  Its things like filter/heater/lights/gravel/water conditioners/etc/etc that make the tank itself look cheap. If you can get a complete tank and some supplies, then you're really in luck.

In my experience, you're far more likely to get fish diseases with new fish from a pet store.  Specialty fish stores are much better in general, but not always.

However, if you do decide to sterilize the tank, use a bleach/water mixture.  That'll kill everything possible, and give it a good wash out aftwards.  That will also guarantee that you'll kill any of the helpful 'cycle' bacteria though.  

If you get a complete tank and decide to clean/sterilize it, make sure you do absolutely everything..  Filter insides, gravel, hoses, tank covers.. the works!  If you skip one thing and it turns out there was a disease present, then the rest of the work is for nothing as that last thing will reintroduce it.

Ultimately, its your call.  If he looks like he takes care of his other tanks and the fish are all healthy, then I personally would be inclined to take it as-is, and be more afraid of introducing diseases with fish from a pet store.