Yes, you can sell used LEGO sets. In fact there is huge market for such sets due to, relatively, high product value even on second-hand market. People use local classifieds for local sales as well as global platforms like eBay, Bricklink for greater exposure.
Could I make a profit legally if it isn't MOC?
It is worth noting, some used sets tend to sell for much higher value than they cost when were new. This is caused by demand, and usually because set being some kind of "special". So yes, you can make a profit.
Would it be against any form of copyright?
You can sell pre-owned vehicles, electronics, household items and this is known to be a norm. How is selling used LEGO different from this?
If they were to be glued together would that be an issue?
Yes, you need to state your model is glued, as it is expected that LEGO model/set can be disassembled for parts.
If I were to make my own website but make credit to LEGO what would be
You need to follow LEGO Fair Play and you'll be alright.