I'd like to allow some users to sell products in my store, without giving them access to magento's back end.
Is there any extension for doing that ?
This functionality is commonly known as an online marketplace. There are a number of extensions that do this, the most popular of which by far is Unirgy Dropship. It is an expensive, commercial, solution.
This has been covered a few times here before:
There's a number of commercial extension available to add similar functionality (search for "Customer as a partner"). I've seen at least two of such extensions, but there could be more.
If you want a freebie, I just installed this one.