The easiest way is probably to get the apps! Taobao and Superbuy are easy to sign up for via their respective apps using your email account.
That depends. Purchasing new often costs about the same as it would locally. You can find used deals on Xianyu/Goofish, but the cost to ship internationally is often extreme (>$100). It's better to source weird items you can't get locally, rather than try to replace your local vendor(s).
There are plenty of guides out there. Being a novice myself, this is not one of them.