It looks common enough but the yellow trailer is the unusual item here. First, you'll only find them in this Gift Set, and not that many of the sets were made. Secondly, there are two types. This is a slightly longer version that has a dip at the front where the Ferrari wheels lodge. The shorter trailer just has two flat panels.
I have no idea which came first or, indeed, whether the sets actually only ever had one type and this or the shorter one has appeared from another source I am not aware of. Both sets have now been sold but I will be keeping an eye out for these trailers now. I also need the other types, carrying a Cooper Maserati (GS6 and GS25) and in the Lotus racing set (GS37) which appear all to be red, but may be in each type too.