Not an unreasonable request at all. It may only be £5 but that’s a lot of money to spend for some people (including myself) to spend on something that isn’t a necessity, plus it will be closer to £10 with shipping and fees. I put important measurements in my description and have in my bio to ask me if you want more measurements. I often feel scared asking people for measurements especially on cheap items because I might get responses like this (I have anxiety) so I do that so other people with the same problem feel comfortable asking me. It’s so easy to take measurements and doesn’t take long, if I’m not able to when they ask then I’ll just do it later. Sometimes it doesn’t lead to a sale but sometimes it does and I’m desperate for money so I’ll happily to that little task to help me make money regardless of the price (most of my items are literally £1). I’ve bought multiple things from vinted that didn’t fit because the seller didn’t provide measurements and I was too afraid to ask so took the gamble and I can never make the money I spent back, I’m lucky if it sells at all. They obviously don’t have to do anything but it would be nice if they did provide measurements when asked. I’d just buy from someone else if I were you.