to say i dont want anything else in a shop do you say baska birsey or hersey istemiyorum. i am getting confused with birsey and hersey.

You say "başka bir şey" (anything else) or "hiç bir şey" (nothing). "her şey" means "everything"

cok tesekkur ederim

what about "bu kadar" as an answer to "would you like anything else"?

"bu kadar" wouldnt be an answer to "would u like anything else?"
u can betterly say "hayır teşekkürler" ( no thanx )