The seat belts on my 1993 Camry are either very slack or not retracting at all. Is a seat belt assembly something I can replace myself or is it something a dealership or auto shop should do?

You should be able to replace it yourself but you will find it is not a cheap item to buy. You usually have to remove 1 or 2 plastic covers where the seat belt and it's retracting mechanism reside and then it's a matter of unbolting 1 or maybe 2 bolts. These bolts will be fairly tight so a breaker bar might be good to have handy.

Mike