I've been selling on Vinted for a few years but before that I've sold on eBay for over 16 years. I've sold thousands and thousands of items in that time.
I have draft listings in my Paperless app stating things like "Please see photos for measurements, composition and washing instructions. All items are from a smoke-free home. I'm open to offers on all items...' etc etc and that saves a huge amount of time (I have several drafts for feedback/reviews too.)
I always research the item I'm selling first and generally list mine at slightly under the lowest price so I hit the top of the search, unless I know my item is in better condition/a rarer size etc, or I think others have undervalued. In that case I list at the price I want and wait for theirs to sell so mine is the remaining option. It can often just be a waiting game.
I take clear photos from all angles including with a measuring tape to guard as far as possible against disputes. I'm very clear in my descriptions and if anything list as a slightly worse condition than the item actually is. I'd rather under promise and over deliver than the other way round - it's not worth the hassle nor risking my thousands of 5* feedback/reviews.
I keep clear records of what I bought items for originally and what I've sold them so there's a clear paper trail in terms of the 'profit' I've made.
I've only ever had one tricky customer on Vinted, I accepted a return from him even though he was telling fibs about the condition of the item as he paid for return P&P and it cost me nothing. I've only had one or two with eBay, and I've found eBay CS to be vastly improved in recent years, it's no longer entirely geared for the buyer as it used to be. eBay has always decided in my favour as they can see the sort of seller I am.
My best advice for sellers is know your item's worth. People do get amazing bargains and there are those to be had, but it's not a race to the bottom. I've lost count of the number of times I've had something listed at a higher price than someone else and mine has sold over theirs. People like to buy from trusted sellers, with comprehensive listings and good photography, who answer messages. It's all about confidence.
I do buy on both but very little in truth, I use them both for sales mainly. My second hand selling pays for my holidays every year!