I received a stamp collection from my dad who got them from his father. There are stamps from many Countries. Is it really difficult to sell them now?
It all depends on what is in the collection. If the collection consists of thousands of cheap stamps (and this is quite common) they will bring virtually nothing. If the collection consists of complete sets of better stamps then it will still be easy to sell even though the stamp market has declined in recent decades. Show some photos of what is in the collection. That way better advice can be given to you.
The albums the stamps are in can also have value. What type of albums were used?