I've spent the last week selling on Vinted and can share my experience!
I went through everything and picked out Joules, Boden, John Lewis, Frugi, Ted Baker etc. Everything else got donated. As I was doing all of my son's clothes from 18m - 6 years that left me with 2 really big bags to sell.
I listed mostly in age bundles to try to shift them in bulk. Individual listings were only for specific items like coats.
I put them on CHEAP. A lot of clothes are listed at prices that are too high because people are thinking about what they bought them for. Forget that. What price would you be willing to traipse to the parcel shop for? That's your price. Generally £1.50 - 2.50 per item.
The other kind of bundle I've had success with is character bundles ie 'paw patrol aged 3-4 bundle' where I've just gathered up everything paw patrol in that age and sold it as one.
Photos of the whole bundle together then separate items otherwise people ask for more photos
Say in the blurb you won't split the bundle otherwise you get loads of messages asking you to sell one item individually etc
Set yourself a time limit of a fortnight or a month or whatever. If you haven't sold it in that time, donate.
It has taken up a lot of my time. I've quite enjoyed it because I'm on maternity leave and I'm enjoying going through all my son's old things and remembering him wearing them. I live really close to the parcel shop though so happy to go out each day with parcels ready to send in the buggy. If it was further (or if I was working) I wouldn't bother.
Hope that helps!