I sold some things on it last year but quickly grew tired of it.
You have to spend a lot of time answering silly questions and sending people additional pictures and then, similar to Facebook marketplace, a lot of people change their minds the next morning (when they sober up I assume). Successful sellers seem to upload new stuff very regularly and contact anyone who favourites an item which is time consuming- getting the clothes out, presenting them, photographing them, uploading photos and descriptions, checking and responding to notifications.
When you do sell, you have to package up, print a label and take it to Evri or another parcel service- and they're all over the place, so that's time and hassle and petrol.
There's a lot of stuff for sale for just a couple of quid - I assume people are selling in big bulk and probably bumping up the postage cost so making themselves a small margin on that. There's also a lot of stolen goods on it, selling brand new with tags for just a few pounds.
I found it pretty depressing to have to sell a gorgeous dress I paid £200 for for something like £10 or £15. Big retailers in every walk of life rely on second hand prices staying rock bottom. No one wants the little person to get much money from selling something on and take that money away from the big companies' margins. So I am a bit suspicious that there might be some more sinister hands at work in the background too.
I'm glad others have had honest success with it, but I really didn't find it worth the hassle for how little return I made (esp if I costed my own time into it all). Good luck with it!