2. Sometimes two or more guests buy items at once
The most common  cause of this is the customers buying shields. In the current state of the in the game, you should avoid selling shields.

6. Skill points for increasing the sales price are invalid.
Charisma bonus is not currently implemented in 1.0 it will be public in the future, for now you should put points into agility or strength as these are the only two working skills.

Some of the stuff you suggested is already on its way in the future.
If you want to look at what is currently planned for the game, take a look here
https://dasius.itch.io/my-little-blacksmith-shop/devlog/54533/a-development-road...