This below information applies to RE sellers and not private customers.

As a private customer or giving as a gift you can do whatever you like.  EG attach 10 charms with different trademarks and give them to a freind.

It has been bought to our attention that some customers are purchasing items that have trademarks and or copyright marks and are then putting these items onto exisitng jewellery and REselling as a complete piece.

Please be aware that this, as far as we are aware, breaks the law.

Examples
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Example A

A charm has a trademark called SupplierA
A bracelet has a trademark called SupplierB

You attach the charm with trademark SupplierA to a bracelet trademark SupplierB.
You created a peice with 2 trademarks and by law this creates public confusion.
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Example B
A charm has trademark called SupplierA
A Bracelet has NO trademark.

You attach the charm with trademark SupplierA to a bracelet without a trademark.
You created a peice with 1 trademark and by law this creates public confusion due to the fact that it would be believed that the trademark SupplierA made the entire piece.
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Example C
A charm has a trademark called SupplierA
A bracelet has a trademark called SupplierB

You have the items listed seperately on your website.
A PRIVATE customer of yours buys them separately and asks you to fit tiogether.
This is fine and legal.

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You may think " Who cares as long as you get your money".
That would be nice but as with any LAWS, we must follow them.
Below are some links to various websites that probably confuse more than help but when you look closely and think about it all, it does make sense.

There would probably be only one in a million cases where creating a derived peice would cause a problem but it has happened with very serious legal consequences. (not to us thank goodness)

It is with regret that if it is bought to our attention that a trade customer is makeing a derived peice then we will have to stop accepting orders from them.

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For the purposes of reselling (commercial trade) you can NOT use a trademark of another to create a derived piece. That is you can not take ANY Trademarked charm, add it to a bracelet and sell it under your own name. This is a violation of the 'exclusive rights' of the trademark. If you mix charms you are in violation of the 'exclusive rights' of more than one Trademark. 
http://en.wikipedia.org/wiki/Trademark_infringement
http://www.ipo.gov.uk/types/tm/t-other/t-usebuy.htm
http://www.ipo.gov.uk/types/design/d-other/d-usebuy.htm
http://www.ipo.gov.uk/types/design/d-other/d-infringe.htm

http://www.copyrightservice.co.uk/copyright/copyright_myths