On ebay, as you're probably aware, some items are on "best offer" - those you can put in a counter offer - although generally you will find this only works for buying in bulk.
You have nothing to lose messaging the seller to negotiate a lower price, as if they are having trouble selling an item, they might relist at a lower price. I find a good way of determining what the market price is by searching completed listings - an appeal that all similar items sell for cheaper or don't sell at that price might help a seller budge. Of course if the current market price on ebay for the thing you want is 15% less why not buy it from someone selling it at market price already?
One word of warning, if they do agree to lower price. Either pick it up and pay for it in person or get them to relist at the lower price. If you try to mail order outside the ebay eco-system the possibility of being ripped off is too high, you lose all buyer protection.
In the UK as a general rule people are not fans of haggling. It can come across as rude and make people uncomfortable, we generally don't/can't do it in most shops. However we do haggle on private sales all the time, such as buying off gumtree, used car sales, etc. - haggling is also more common in service industry roles, especially when negotiating a service.