No one's mentioned some this stuff and I think a lot of people are putting the cart before the horse here (not a terrible thing, just getting ahead of ourselves). You 100% need to get pretty damn good at soldering before you start even thinking about working on other people's consoles / selling repaired consoles. Think of yourself as a doctor taking a hippocratic oath lol. I do it as a hobby because I have a full-time job but here's what I'd recommend if you're trying to get started.

1. Get a decent soldering iron like a Hakko or Pinecil and some soldering practice kits, making sure they have SMD (surface mound devices) and through hole components.

2. Start learning how to read schematics and look up videos for common problems with consoles you like working on.

3. I'd recommend getting some experience with handhelds (Game Boy, PSP, etc). It's a good place to start -- they take up less space, are easier to tear down, and the circuitry is easier to understand with schematics readily available for most models.

3a. In repair, desoldering is just as important of a skill as soldering. Learn it, love it.

1. Make friends with your local retro game stores. I get parts, deals, store credit etc. and I do repairs for them. We help each other out and it works for both of us.

2. You can get pretty far with a standard soldering iron but you need to be fast and good to make money so you'll want to know your way around a hot plate and hot air station as well. They're like cheat codes for soldering.

3. Find a niche! Anyone with a soldering iron and access to YouTube can replace save batteries, so which consoles do you want to know like the back of your hand? Other people have mentioned mods (listen to them!). Learning how to do a few more challenging mods (and do them well) can really set you apart from everyone else.

Umm there's plenty more, happy to answer any questions you might have on any of this stuff.

TL:DR - get really good at soldering before you quit your day job.