Go pick a random figure. Play around with it for a bit. Transform it, try out any gimmicks. Put it down and think: did this toy work for you? Did it make you happy? Does it make you think "well, it's good, but..."? Do you just kind of stare at it blankly?

Really if it's anything other than the first answer, it's not a figure you need. As someone who has gone from trying to get pretty much everything to just specific figures I want (basically anything that can fit in my Classics or can be repurposed as such), and while making that decision and parting with some toys has been very hard, I haven't been happier as a toy collector in a long time. It's less stressful, takes up less space, I spend more time with each figure, and it doesn't cost anywhere near as much. Selling off your whole collection will no doubt lead to a lot regret, best thing to do is streamline it first. I know you probably don't want to hear "well, take it slowly", if you need money quickly but that's the best thing to do. Don't be rash.

And, yeah, TF aftermarket prices are crazily inconsistent, so good luck with that.