make a random offer and see if its accepted. most sites i use dont allow 'offers' all parts for sale have to include a price. so id haggle on that price.
or be a bit smarter and post a "ive been looking for a part x for ages and someones offered me part x for £xxx (make up your own guess price), is it worth it ? what should i pay for that part in that condition? " instead of a "whats it worth" thread
thats what i did and it wasnt deleted