I mean, there's no right or wrong answer here, is there? It totally depends on whether you need/want the money you'd get from selling them. If you're in a position where you can buy a £350 jacket and then not wear it, my guess is that the money isn't really a big deal to you.
(Although I'd be surprised if you only got £30 for that brand new £350 All Saints jacket, by the way! I've just sold a Doc Marten leather bag for £50. It wasn't new and the current in-store price is £120.)
I personally wouldn't bother trying to sell things from places like H&M, New Look, Primark, Asos/Boohoo etc, because they're so cheap to buy new. Remember that if you price a top on Vinted at £5, the buyer also has to pay a couple of quid postage and the buyer protection fee, so they're more likely going to be shelling about £8-9 altogether and they could probably buy a top new from Asos with next day delivery for that. So you end up having to reduce things to £2 or whatever and for me, £2 wouldn't be worth the effort and I'd rather just take the item to the charity shop instead.
Generally, my own rule of thumb is that if I'll make more than £5 for it, I'll sell it, and anything else goes to the charity shop or gets recycled. The sorts of prices I've got for things in the last couple of weeks have been £45 for nearly new Doc Marten sandals, £50 for the bag I mentioned, £25 for some barely worn Birks, £25 for a new with tags Roka backpack, £25 for barely worn Converse, £12 each for two pairs of leather ankle boots worn a few times, and £6 for some Levi shorts. Obviously these prices are nowhere near what I paid for them new - but I'd rather get something back than nothing and while I'm not on the breadline by any means, I could do with a little extra cash at the moment so it's been well worth my while.
Obviously season helps as well - you'll make a lot less selling (eg) coats and jumpers now than you'd make if you were selling them in September.