Hi miss-cheivous,
You need to consider two different aspects of intellectual property: copyright and trade marks.
Copyright will exist in drawings of cartoon or anime characters which are already in existence, such as say, Miss Kitty or Bart Simpson, but if you create your own new character then there would be no problem, but I assume that is not what you want to do. To be able to legally use copies of such characters you would need a licence from whoever owned the copyright, usually a big studio like Disney or a publisher behind the comics. That may well be what other producers of such jewellery have done, or of course they may just be hoping to escape notice. That is assuming that you are making copies of the characters. If however you purchased existing licensed images such as the sheets of stickers with the characters on available from newsagents and pound shops etc, you could use this artwork freely without infringing copyright as you would not be copying anything, merely using the original item which was already licensed (which it would be reasonable to assume was the case if you obtained them from a reputable source).
Trade mark restrictions are somewhat different. Most of the popular cartoon and anime characters will also be registered as trade marks, and you need to be careful that in using them in your jewellery you do not appear to be trading on the goodwill and reputation of the trade mark owner or passing your product off as a product originating from that owner. Of course, it could be said that the sole reason you would want to do this is to trade on the popularity of the characters involved, otherwise there would be no point in doing it, so that leaves you with a difficulty unless you get a licence to use the trade marks. You can read a bit more on this topic here:
http://www.copyrightaid.co.uk/forum/topic1166.htm
As for you final point, there will always be traders who chose to ignore the law and hope to get away with it, just as there are drivers who exceed the speed limit or park on double yellow lines!
