Re: not sure this is the right place :O
Bearing in mind the over-optimistic way that many internet auction sellers describe their lenses when selling, and even if you buy at a shop the lens may look OK but you don't know how many times it's been dropped, abused etc...
I'd say that if you know the person selling this lens and trust that they have looked after it, it's worth paying what some consider to be a little OTT for a private sale, if only for the peace of mind in knowing that the lens should be faultless.
How much more could you spend in looking round to get another lens in very good condition? Not just cost in travelling, phone calls, but in your time?
As to whether or not you need it........ well, look only at my signature text.......
Too many cameras, too many lenses.......