So here's another thing I've hardly seen any guides on on the internet; what are red flags when buying a bike? Like, apart from the general stuff that comes with buying anything.

For any of these points, it doesn't really matter if you buy used or new, the point is if you're buying cheap. There's nothing wrong with getting a cheap bicycle. But much like cars, if it's cheap and flashy odds are something is wrong with it and you'll hate it. If you're looking for something to get around in, you do not want the 1990s honda civic with the lamborghini bodykit conversion, aye?

So here's what that is on bicycles. I'm excluding E-Bike specific things as I have little knowledge of those.

Anything with wheels that doesn't have regular spokes

I'm assuming everybody knows what a run of the mill bicycle wheel looks like. If it does not look like that but only has like 3 massive spokes or is just a full plate and is also cheap, stay the hell away from it. There's a reason you don't see those often and that reason is the good kind of those is expensive to make safe.

And you do not want your cool wheel to just disintegrate at 20kph

Cheap Folding Bikes

There is good, cheap folding bikes. If you do not know your way around one, the chances of getting one of those compared to getting one that folds when you do not want it to is not in your favour.

Seriously, if you want one, and they are practical, do some research. Or ask someone knowledgeable. Especially when it comes to folding mountain bikes.

Full colour tyres

Tanwalls are a different thing, but if they're one solid colour that isn't some black-grey thing stay away.

It's the same deal as with the folding bikes, there is okay coloured tyres, I have a bike with one.

The chances of you getting one of those if you're not quite into it is, unfortunately, rather slim.

Either stay away or invest in some other ones. If you have a bicycle, knowing how to change the tyres is sort of a must anyways.

Misspelled brand names

If you can see a brand name, type it into google. If google tells you "Did you mean ...?" stay away.

This isn't to say there isn't good cheap brands or anything, do not get me wrong. There is, even if many other cycling communities would disagree. But you do not want parts that try to trick you into thinking they're other kinds of parts.

it looks like a can of monster energy with two wheels

I hope this makes sense, but if you see one and it is super flashy stock and not like, custom modified, odds are this is being marketed to people who's first need is cool and "actually riding it" isn't on the list at all.

It's full suspension

If it has a front suspension, fine, many bikes have those.

If it has a rear suspension and is cheap? Stay away. The best case scenario here is that it's heavy as sin and doesn't do anything. Worst case scenario is it just falls apart.