80 figurines is a lot, especially if they're stacked without any organization. Before selling, I'd suggest really sorting through them: keep the ones that meant something to you or that you actually love, and be honest with yourself about the rest. Once you've got a solid selection, the space becomes way easier to manage. The ones you keep will bring you more satisfaction than a pile of stuff you don't even look at.
For organization, wall shelves are really the best investment. They don't take up floor space, they won't make your girlfriend feel like the place is being taken over, and honestly Funko Pops are made to be displayed, not crammed together. Small shelves around 60-80 cm are plenty for a reasonable collection. Otherwise display cases with doors are cool too - they keep dust off and look really nice.
If you sell part of it, resale sites work pretty well, but check actual prices before you get your hopes up. Some rare editions are worth something, but most Funko Pops are basically worthless. Thing is, you'll probably barely make anything and it'll take time. Sometimes it's easier psychologically to just give them to someone who'll appreciate them rather than struggle to sell them for three euros each.