You’re a selfish, aren’t you?
Is this grammatically correct, and is it how a native speaker would say it? The context is that person A continually acts in a selfish way, and now another person B (a friend or colleague) who has been around him for a long time asks this question in a situation where person A says something selfish.
This is not gramatically correct, no.
“a” implies a noun, e.g. “person,” “woman,” “worker.”
“You are a selfish [type of person]” would work, e.g. “You’re a selfish man, aren’t you?”
If you do not want to use a noun (because, for example, you don’t know what kind of person they are because you are talking over the internet) you can instead write “You’re selfish, aren’t you?”