I have a 1950 era rowhouse, with 100 amps and fuses. New York City. I
haven't had any issues with the power and all the fuses are 15 amps.
There is on 240 volt circut for a large AC unit which is just two of
the 15 amps combined somehow.

Is there any need to upgrade the electric before selling the house?
I've had some people tell me that 200 amps and circuit breakers are
now required. I've hardly ever blown a fuse but can do so by running
the toaster oven at the same time as the microwave. Not always, but I
avoid doing that. In fact I replaced that fuse with a circuit breaker
dressed up as a fuse, which screws in but doesn't need to be replaced
if it pops.

There are two 100 amp fuses in the fuse box (big guys), so perhaps
there is already 200 amps coming in and just 100 are in use?