Some gear is easier to buy used than others, especially for a beginner. Stuff that needs to fit (mask, BC, wetsuit) is difficult to buy remotely unless you have some way of knowing a specific item will fit.
If you find a suitable (low profile, tempered glass) used mask that fits, great, as long as it's not ancient with the skirt starting to get stiff. Straps can be replaced (many people replace them with something like a slap strap anyway.) A used snorkel is easy, though for diving it's best to avoid ones with giant dry tops as they will pull on your mask and are more difficult to stow elsewhere. Used fins can be a great buy, and anything starting to wear or bad straps (easy to replace) should be pretty obvious if you can see the item. Wetsuits age, wear, get compressed, and suffer other indignities, so not a great bet used unless you luck into something that is pretty new and gently used.
A used BC can be a good deal and is fairly easy to check out yourself (been discussed on other threads.) The older they get, the more likelihood of leaks from the bladder, especially if it was stored carelessly, so watch out for that. You will actually want to inflate it and leave it sitting overnight to see how it's doing.
Buying used regulators works best if you really known your stuff (or at least get help from someone who does.) And ideally if you will be servicing them yourself, which most people don't. Unless you buy a used reg that was just serviced from a shop or someone you trust, you will have to get it serviced. This is generally not cheap and can eat up a lot of your savings as compared to buying a new mid-range reg. While an old reg should be functionally as good as new once it's serviced, it will still be an old reg that you could wind up putting just as much money into. There are lots of old regs that are popuiar and as good as new ones. But there are also some that are difficult if not impossible to get parts for, or at least not worth the trouble for most people.
Prices on craigslist and similar can be all over the place. Some will price too high, trying to get much of their money back on something they overpaid for 10 years ago. It may also be hard to get accurate information from sellers because they got some gear who-knows-where and don't really know anything about that they're selling. There is a lot of crap out there.