I'd look somewhere else to be honest. If an established snake only eats live and only eats every 3 months then it's going to be hard to get it on frozen/thawed. Some particular snakes just go off feed every winter, so if this one has done it in the past, there's not much you can do in the future.

Also, feeding outside the cage is another one of those preferential things. Some people do it, some don't. One isn't better than the other. In fact, I think it just stresses them out even more (which could be bad for a snake that only eats a few times a year anyways..)

There are tons of other places than craigslist to get a ball python (pet stores, breeders, other ads). Just get a hatchling and raise it yourself, that way you will get to know its eating habits and start it on F/T rats early on.