(Based on my previous experience with searching in large databases, but not specifically eBay as they do not share their code AFAIK)
When new items are created the system prepares by taking the item text breaking up into chunks trimming punctuation and then building a matrix for later use such that it can find things despite misordered words and partial matches, dealing with plurals and other pre- and post-fixes.
When a query is presented it again breaks up that text and trys to cross match the item chunks - in this way search-time does not grow a the same rate as the number of items which are listed.
Where you have non-words which may or may not be part of references (part codes) it can have a lot of trouble esp where they include spaces as these may be tokenised such that the letters are separated out from numbers .
In the case of the Ring light I suspect that the tokeniser has left out the 'x' so it will find it if I also leave out the 'x' (pic2)
or
if I include quotes around both terms it will find it successfully. (pic)
Although this is not always 100% guarantee - and generally it is better to include something which is a strong discriminator in the search e.g. a brand name or a category to give the searching more of a fighting chance if you can.
