When Frank was talking to the car dealer in Ocean's Eleven, he specifically mentions his soft hands, resulting in a better price for them for the car. Why? What is the background?
Watching the scene it seems that the talk about soft hands and lotion is mostly to give an impression to the other people in the store that nothing is wrong.
Notice how the dealer becomes very uncomfortable, starts stammering and clutching his hand and how he rubs it after Frank finally lets go. He's no longer a confident salesmen at the end of the talk. He is scared of Frank and is willing to lose a lot of money to make him go away.
It's not about "discussing a better price", Frank is intimidating the salesman by holding on to him, crushing his hand and the way he is talking (not the words, but the way he says them), while keeping up an appearance to onlookers that it's just a friendly discussion.
It's more like keep the conversation going. Also you can notice that he talked in lower tone. So, it was just politely talking with the car dealer and making him take the price down.
Talking about lotion has nothing to do with the conversation. It was rubbing the hand which possibly made the car dealer to get rid of this quickly. So, he lower the price and Frank's trick worked.
FRANK : Yes, Denham, like a jean. (they shake) You know: you have lovely hands -- do you moisturize?
BILLY TIM: I'm sorry?
FRANK: I swear by it. I try all sorts of lotions. I went through a fragrance-free period last year, but now I'm liking this new brand fortified with rose hip. My sister, you know, she uses the aloe vera with the sun screen built in...
BILLY TIM: Uh-huh. You said you'd be willing to pay in cash?
FRANK: I did. You know: they say cinnamon is wonderful for your pores. Read that on the internet. And that ideally you should be wearing gloves to bed, but I find that would interfere with my social agenda. Problem is: I get a reaction to camphor so I can't use traditional remedies...
BILLY TIM : If you could pay cash, I could probably drop the price a little. To, say, seventeen... (as Frank squeezes a little) ... sixteen each.
FRANK: That would be lovely.
Including the lotion in this conversation just makes it silly and Frank just did this to politely force him to lower the price.
As others have said, it's an intimidation tactic to lower the price of the vans Frank wants to buy.
In America it's socially abnormal to hold onto someone's hand after shaking, especially in a business setting. By bringing up "soft hands", Frank essentially has an excuse to keep holding the salesman's hand during the conversation. While outside observers would see and hear them talking about soft hands, he's actually squeezing the salesman's hand to motivate him to lower the price, which is made most clear at the end of the scene when he intensely squeezes and the salesman drops another $1000 off the price.