Having read this thread there seems to be several questions, not just one, and that is causing a lot of confusion with people saying - "but what if" and then getting an answer to the wrong question?
There is also a clear difference between a private and retail sale, regardless of what you think, and private sales are not covered by much in the way of law at all, certainly nothing beyond what has been placed in an ad as the "description" - if it's broadly correct and not deemed to be deliberately trying to decieve you then your stuck - someone’s "good condition" may not be your "good condition"?
With private people buying and selling like this they are forming a "contract of sale" - as they have very little in the way of rights on this to begin with, then what is written in the sales ad forms the basis of the contract - so if a seller says "will post at buyers expense and risk" then by agreeing to a sale the buyer has accepted this, and this is their contract, and they are now liable, whether they like it or not. If they don't accept this, they simply do not agree to the sellers terms and do not buy- easy!
Evil bay and PayPal don't do guns full stop, they will not help in any way regarding a gun transaction as you have broken their Ts&Cs if you have used them for this. The bay will help with components or accessories depending if it meets their criteria - you would have to read their very large amount of small print for this.
I'm sure some people will agree with what I have written, others won't.
There is also the big question of what is posted - guns and expensive scopes are the big thing, and most people will understand and accept this has to be agreed to liability etc., but there are then the other items which have less value and are more "postable" in the normal meaning of not being on any excluded or prohibited list - a bi pod for example.
Most people will do the right things with smaller, less valuable items, or get it insured if it can be, but there are scammers on forums whether we like it or not, and there have been threads to prove it. So I think it's in everyone best interest to look at the item, the seller, the buyer, and how many posts they have, length of service etc. and make an informed decision as to whether they want to do business - which is what we all do subconsciously to an extent, we should just think a bit more about it as the cost and risk rises.
I don't think there will be or can be an answer to this thread - at least not in the thread it's self. I think it's just a case of knowing what your getting into - which when spending the kind of money we see changing hands for some items is sensible if nothing else?
James