It helps if you learn to translate Customer, and learn how to speak Anti-Salesman.
Speaking Salesman is like talking to a normal person, but that person expects you to do or say anything to convince them to do something they don't want to do.
Anti-Salesman is a way of speaking that counteracts this. My managers didn't care for it, but it worked for me. "Hey, s'up? What brings you in. Shoes? Well we have a lot of those. Which shoes?"
It also helps how to filter Customer back into a language that isn't one spoken by someone who expects you to do or say anything to convince them to do something they don't want to do.
"No questions." -> "I have a ton of questions."
"I don't want that." -> "Thanks for the recommendation but I have something else in mind."
"I have a quick question." -> "I am about to waste your time for half an hour."
"I have a stupid question." -> "I thought about this already and I'm stuck; please help."
"Just looking." -> "Keep an eye on me, because I could have a ton of questions I'm just not ready for them yet. I could also be just looking."
If I can make the suggestion, here's what worked for me as a follow-up:
Customer: Do you carry BearClaws?
Me: No, but we do have shearling footwear by <brand>, it has <brand feature> and we're running a sale--I can grab you a size to try."
Customer: "No, I was looking for BearClaws."
You: "What about BearClaws were you looking for?"
Customer: "I can't find anything else that fits my foot as well."
You: (knowing about BearClaws) Is it the arches?
Customer: Yeah, <rambling story about how when they were a kid they were frightened by a McDonald's and therefore his feet lost their Golden Arches>
You: Huh! Well maybe try these MonsterFeet boots on for size. They've got great arches for some people.
And if it doesn't work for them, then at least they tried.
This assumes you're on commission. If you're not on commission, it's super-busy, or you just don't want to deal with this person, then skip the advice. Making five-minute friends in a retail environment is not impossible. As a light introvert, it's I think it's kind of fun.