"the guy I bought it from said"... If I had a nickel for every time I heard that. Hell even my super clean 98 I got had some surprises for me but you have to expect that lol.
The T-5 will have nothing to do with an oil leak, if its leaking, it'll be trans fluid. You sure its oil leaking or is it Mercon? He probably did do the rear main seal, doesn't mean he or the person who did it did it right. Its very easy to not seat that seal all the way or think you did and still have it leak.
I hate main seals... $5 part and like 3 hours of labor on a lift with a friend if you are good. Don't even get me started on what shops quote to do this...