@Nikola, I consider a lowball to be anything under 10-15% of the market price of the item. https://en.wikipedia.org/wiki/Low-ball
Best source of market price = eBay. Try to find an item in the same condition(NWT, etc.) by the same maker in a similar size/color/fabric
Obviously, if it's a staple item in navy or grey or something, you'll have less leverage to negotiate with the seller on price. It will be an item that many people want.