I have a large collection of Watford badges dating from the early 50s and being a supporter since 1963 i know nearly all of the official badges issued by the club. Some badges sold on ebay are not official and although some look official they are a bit dubious. As one example of badges that are not official club issue are described as "with plastic insert" ( these badges may come in different shapes but the insert is nearly always the same and they look cheap ). I can produce pictures of most of the official badges for you to look at and i can give you some other info that may help you avoid unofficial or fake badges. I may also be able to help with other info on Watford i.e. autographs etc.as i have large collection of these aswell.
    Another area i can help is with Bristol blue glass, too often i have been looking on ebay for more items to add to our collection and items titled Bristol blue glass have turned out to be nothing more than ordinary blue glass. We only  have a small  collection but i can give you some tips on how not to get caught out.
   The general rule to buying Bristol blue glass is to check if it has been marked on the base, if it is etched on the bottom with Bristol then chances are it is genuine, most Bristol glass is marked. But like everything else these days there are people out there that do their best to rip us off with fakes. Also if it's claimed to be old glass then check the base for pontil mark and see if there is any wear on the base too. Its not an exact science buying Bristol glass but if you email Bristol blue glass they have people there who are willing to answer any questions you have, this is how i started out and since then i have not bought any dubious glass.
    Lastly as with buying on ebay, look at sellers reputation and feedback and ask questions. A genuine honest seller will help you with any questions you have, if their answers are slow coming or a bit vague i leave well alone.