People asking for measurements annoys me, because I don't have a dressmakers tape measure! Yes if you were buying it new you'd be able to try it on in the shop/send it back, but you'd be paying 40 quid for the jeans in the first place- if I'm selling my old new look ones for a fiver I think you can weigh up for yourself if they'll fit and take your chances.
An annoyance about Vinted in general is how they categorise their sizes- as someone said upthread, a 14 is an XL and its hard to know sometimes how to categorise stuff you're selling. Its definitely resulted in me buying stuff that turns up a totally different size to expected before.