I would suggest NOT allowing the skins to be uploaded by the portal.
The portal 'administrator' should email a requested skin to you and allow you to test the skin on a test portal so you can review it beforehand for security reasons.
If the skin meets your requirements, you can have payment made to you to allow the skin, then you can move the skin and containers to their portal. Once that happens they will be allowed to use it from the Admin | Skins page since DNN will automatically recognize the skin in their portal folders.
If they don't pay you or the skin is not secure, they can't use the skin.

- May I propose all my skins in Admin -> Skins tab and pay only one license for each skin bought ? Term of use doesn't mention anything generaly.
Normall as long as it's one DNN installation, yes.

- If a customer change his skin by a new one, do you think page render is beautifull immediately ? or skin need html adaptation ?
If it's a good skin it should, but there might be some small changes to make for specific modules

- Does a marketed skin always propose a place for the company logo ?
No, most do though, the skinner is completely free in this.