Maybe slightly less figures related but I had a point when I sold entire collection of kpop albums and stuffs, first I was narrowed it down to left only few band I like, at the end I end with only 3-5 albums of two bands I liked the most; first time it was do it or not do it?! But I already was one step into figure collecting, so for me the motivation was thought how much money I'll get back from sth that sit on shelf and doesn't even look good (I didn't use my albums except for collection purpose and looking at photos) so slowly, while selling and see money increase and thinking which figures I finally can get for this, I let go off cd's. Simple I realized if I don't enjoy it anymore there is no point in keeping it.
You can try put all unwanted/ to sell figures in one display and try to look at them for a while and notice if u still enjoy them maybe sth will click if not you will be more sure to get rid of them.
For me few big factors in the past when I sold some figures were overall look at my collection, space, what I like and sometime getting one more expensive item that bunch of smaller which I knew I won't enjoy that much.
For prize fig without box I know collectors on MFC are often strict about it, honestly I would buy prize without a box (I did it twice from ebay and I don't regret it), if seller know how to pack item safely and they put slightly lower price. So far I sold 2-3 prize figures but mostly in my country on auctions so "no box" wasn't an issue for most ppl, they simple didn't care about it.
In my experience box issue is big if it is nendo or scale, prize is less important, especially that often box don't even protect figure itself, unless it some kuji line or sth.