I buy mostly clothes for DCs, but also sports equipment for them such as football boots and swim fins. I buy all their training swimming costumes on Vinted and they are rarely not immaculate (competition suits are bought new). Occasionally I buy shoes, clothes for me and toys.
I sell everything the DCs outgrow that will raise more than £1-2 after having also passed on a good amount already to friends. I find supermarket brands don't sell, even at £1 when in perfect condition, so I hand these down along with anything we were lucky enough to have handed down to us that is still good - it doesn't feel right to make money on that kind of gift. So mostly clothes, shoes and the aforementioned equipment. Coats sell particularly well.
I have also been trying to clear the backlog of outgrown stuff from Covid times when I wasn't able to sell at baby sales, so everything just got bundled into the loft. I have had success with baby towels and bedding, Gro bags, and phonics scheme books.
What rarely seems to sell on there are bundles, whether listed as a bundle or buyer-created (to take advantage of multi-buy discount and combined postage). I used to regularly buy smaller bundles of baby clothes when mine were tiny and thought they were great as they were often carefully put together to make an outfit/capsule wardrobe or all be of a similar taste. There was fierce competition for them locally on FB Marketplace! I always check the seller's other items before buying on Vinted and will often add additional items, but I wonder if others don't always know this is an option?