successful in / at

Hi there, imagine someone has got a shop.
"He is successful in selling gum"
"He is successful at seeling gum"
Can I uses "at" and "in" in both sentences?

How do I express that someone is successful "with" something.
For example: "He is successful with his shop" sounds wrong to me.
Can I only say: "His shop is successful" in English?

Thanks!