Book shops accepting second-hand books

Pele Mele in the centre of town will look at books in all languages and buy ones they don't have lots of copies of. To give away, most of the chrity shops like Oxfam will take them

To some extent it depends where you live.
French books will be less attractive if you are in the Dutch speaking part of the country and Italian books may not be wanted anywhere.
Pele Mele have branches in Brussels and Waterloo and there are other second-hand books shops in the same area of Brussels as Pele Mele.
My experience with Pele Mele is that you will barely cover your travel expenses with what you will get for your books.
If 'levels 1 to 4' implies that these are educational/language learning books, you may find that a local school will be interested in them and I have found that my local commune library welcomes donated books.

Dear Paul,

In case you would like to donate - I'm collecting French books to set up libraries in RD Congo - Of course I can come and pick them up.

They go to the organisation "Kulture Sans Frontière" - you can find them if you do a simple search in Facebook.

Have a nice day

Anja