Under the 'rules for sellers' section, eBay has this to say about it...
Keyword Spamming Â What is Keyword Spamming? Â Keyword spamming is the practice of including brand names or other inappropriate "keywords" for the purpose of gaining attention or diverting users to a listing. Keyword spamming is unfair to members who may be searching for a specific item and receive search engine results of listings that are not selling the item. It is also a manipulation of eBay's search engine. Users often are confused and frustrated by such tactics. Certain uses of brand names may also constitute trademark infringement and could expose sellers to legal liability. Â eBay Policy on Keyword Spamming Â Excessive use of keywords, including (but not limited to) brand names, which are referenced for the purpose of attracting or diverting buyers to a listing is considered keyword spamming and is not permitted. Â Examples of Keyword Spamming Â Here are some examples of keyword spamming that are not permitted: Lists of words. (1.) The inclusion of words that are simply listed to attract viewers via our search engines and are not part of a context based item description. For example, it would not be permissible to state the following in your item description: "oil reproductions, art recreations, gallery, art gallery, impressionist, impressionism, oil paintings, reproduction, painting, recreation, copy, quality, reproductions, recreations, realistic, copies, paintings, old masters, replica, posters, prints, video..." (2.) The inclusion of words that are simply listed to divert viewers to your other eBay items. For example, it would not be permissible to state "Please view my other eBay listings for Beanie Babies, vintage Barbies, Rolex watches, and Chanel scarves." It would be acceptable to state "Please view my other eBay listings." Hidden text. HTML included in the listing, such as hidden text (for example "white on white") or source tags (meta or header tags) are not permitted. Drop down boxes. Inclusion of a drop down box in order to provide buyers with a list of items offered in other listings. Because the search engine will pick up words listed within the box, this interferes with the search results, and is thus not permitted. As an alternative, sellers may include drop down boxes on their About Me pages, as this would not interfere with the search engine. Certain uses of brand names may also constitute trademark infringement and could expose sellers to legal liability.
I guess the main point here is 'excessive'. If you had, say, a shirt that was made by the company that makes shirts for Brioni (is it Baroni?), then you could probably legimately say, 'NWT shirt, like Brioni'. You couldn't say, however, 'like Brioni Kiton Armani Gucci', because it isn't.