Have to agree that this seems legit enough to be okay for me. You did quite a bit of legwork and played in a specific way to get that overpowered. I'd argue that circuitous methods like that are part of what make these games fun.
Yeah, sneaking past a good bit of content then returning with a strong weapon seems fine to me as well.
However, I would say that axe is probably too strong to be sold by that vendor at the time one would naturally arrive at him. Moving him more towards the end of Act 1 would probably make more sense.
Also, the fact that there are not equivalently offered weapons of the other types anywhere in Act 1 is pretty disappointing.
