In trades I really need to know if is correct the use of this idiom. If its ok or not, would you tell me what are the common ways to express when we need to buy or sell some goods when the buyer and the seller are face-to-face? Please, teach me the formal and informal ways to treat someone properly (without being rude).
No - "hand to hand" has a different meaning, and refers to a kind of fighting, ie hand-to-hand combat. "Changing hands" is a term used to describe the sale of something from one person to another, but is mostly used to describe the sale of businesses themselves rather than goods. The term "second-hand sales" also refers to the sale of pre-owned goods.
Any kind of meeting in person can be referred to as "face to face". Face to face selling, or face to face retail are expressions used in the sales industry, as is personal selling, all of which describe meeting with clients to make sales.
There are other terms for selling in specific locations - for example, "high street retail" refers to selling from a shop or store.
I think the phrase you are looking for is probably "face to face sales", although this is more of a literal description of what you are doing rather than an idiom.
I would certainly not say hand to hand, as that implies unarmed combat. However, I also wouldn't say that buying something face to face is a natural expression either.
Instead, this would be a far more natural dialogue:
"Did you order it from him on the phone?"
"No, I bought it from him in person."
Alternatively, you could say that something is an in-person sale.