set aside enough of your favourites to last you till they expire. everything else will expire before you even get to use it, if you even want to use it, hence they're pretty much worthless and you've already actually lost the money anyway.

whatever you can recoup by selling the excess is the most you'll be able to make from those losses, plus at least someone else gets to benefit from them instead of letting them go to waste! see it as getting paid to get declutter them quickly instead of letting them take up storage space till they expire and become unusable anyway. if you haven't gotten round to using it, chances are you don't have enough time to get around to using it AND testing it long enough to realize it's what you 'really' need before it, or majority of things, expire.

if what you've currently had works for you, then don't fix what isn't broken. if you really want, you can try decanting some into a bottle and selling the rest, but people will pay less for an opened/partially used item and besides, you should only introduce new skincare products every few weeks or even months to ensure that it doesn't cause an adverse reaction. you can always experiment with new things when you've finished with your current item.