It's not rare-rare in that it is uncommon. It sold a lot of copies, even though it's one of the earlier Psygnosis titles. An example of older Psygnosis titles from the same era that *are* rare would be Deep Space or Arena, which were far less popular and thus relatively obscure (and command a higher pricetag in online auctions).
Another factor which makes Psygnosis titles rare is the inclusion of extras, such as t-shirts - particularly if they are unworn or if the package still shrink-wrapped.
Despite these factors, you may be able to convince the person you are dealing with in the trade that the game is more rare than it actually is. I have even seen ebay auctions with text that suggests something rarer than it is and the bids increase accordingly, but not usually.