The thing is and I hope this is not an actionable post but, inflation happens for many reasons, people can artificially inflate like you were suggesting but also happens because of the lowering of the RMT value of an in game currency like gold in UO. When it becomes less valuable, prices go up. When I first started making gold I was investing in items that I thought would beat inflation in terms of appreciation in value because what was once 3p five years ago may be worth something towards just less than double that now. Unfortunately prices do go up on a lot of things but you have to look at it how I look at it and how I think you mentioned in your post that the price of your collection has also risen. My main comparison is with [exceptional](blessed) items with many of these worth towards the plat mark especially skirts or equivalent slots, cloaks about the same and footwear even more. You may have noticed some of these type of items on safes recently at 9p or so, personally I think that IS ridiculous but with this item that is what I’m comparing to as really it’s just a rung down in terms of rarity to those.
But like I said, test the water, make a fair offer and see what the OP has to say. Prawn may outbid you but who knows, maybe the seller just wants a quick sale or maybe he isn’t even looking to sell. Your first response seems to suggest you wouldn’t even be unhappy to pay 400 or maybe I read that wrong.