I think on Ebay these days you can make an offer even if the seller has not included "or best offer". If you go to "contact seller" and push through to sending a message, you can make an offer there. Ebay seems to encourage it! Sometimes (I can't quite work out which sequence of events brings it up) a button for "make an offer" appears.
That seller is in the US and prices there consistently seem much higher than in the UK, even for GB coins, so I would not be surprised if they get 2/3 of the asking price on most of those. As Rob says, it is much easier to drop your price if you have over-cooked it than ask for more once the buyer has clicked buy-it-now!