Just a few points...
We sold one of these on ebay in February this year, and I've seen at least two others on ebay in the last 12 months (without specifically searching for them) so it isn't three years since the last one on ebay.
The signature isn't "carved hand script"... despite some online claims that it is.... it's an acid etched or sand blasted facsimile signature.
I understand they were originally designed for the 'Coast' restaurant in London circa 1995. I think I read somewhere that they were going to be some sort of special plastic but there were production complications, but you should check all these details if you want to include them as I'm just writing from memory.!!
There's a photo of the Coast restaurant here with green versions of the ashtrays on the tables: http://mydeco.com/buy-the-look/1585/
A google search with the words "coast", "urchin" or "blend" may turn up some more info and examples.
Best of luck with the sale.