A few things I would like to point out about this set, and also collecting Adams in general.
1) The first UK editions of the first three books were the paperbacks from Pan. The Arthur Barker hardbacks were later, but they were the first hardbacks. By the way the first printing of the Pan paperback is different to later printings.
2) The Life, the Universe and Everything for sale is an ex-library edition. Such editions should be worth much less than a one that does not have library stamps. However, the point of the Adams / Arthur Barker hardbacks was to supply libraries which wanted the longevity they offer, so unfortunately there are very few without these stamps. Before buying this I would want to see the condition as ex-library books vary considerably.
3) Judging by the descriptions these were recently offered on ebay, see item #380111260466 where you can see a picture. I would describe the fading on the first book as severe. The picture is also on AbeBooks - http://www.abebooks.co.uk/servlet/BookD ... 1301495864
4) The fourth and fifth books are common and you are probably paying a premium for them by buying with the other three. Personally I find it is more fun to build a collection slowly and also probably cheaper as you can spot individual bargains.
I think there is a reason the collection did not sell on eBay, because in my completely amateur estimation I would value the collection at just half what the seller is trying to get for them - possibly less depending on condition. Having said that you pay your money and you take your choice.