“Oh yeah? What kind of sales?”
You're at a park with your children. While they're playing, you're talking with one of the other parents. He says that he's in sales. You're interested, so you ask for more details.
Oh yeah? What kind of sales?
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This is a way to respond when someone tells you something that you didn't know:
A: I'm competing in a 10K this weekend.
B: Oh yeah? I didn't know that you were such a serious runner.
You can think about "Oh yeah?" as meaning "Is that true?" This is a friendly, casual phrase.
"What kind of ___?" is a way to ask for more details about something. For example, if someone tells you that they like hip-hop music, you can ask:
What kind of hip-hop?
You're asking to hear more details about the specific genre or style of music.
As another example, imagine that you're talking with a coworker about what food he likes to cook.
A: I do a lot of baking.
B: Baking? Really? What kind of baking? Like bread, or like cakes and cookies?