As an eBay Top Rated seller who is very familiar with Links of London products, I thought I would share my knowledge and experience to help you tell whether the LOL Charm you wish to buy is the "real thing" or a fake.
Very generally, you can pretty much guarantee that an eBay Top Rated seller of Links of London products will be selling genuine charms. This is because no seller could reach the high scoring targets required by eBay to reach this status if they sold fakes. Additionally, a seller obtains discounts off ebay selling fees once they are a Top Rated Seller and no one wants to risk losing these discounts.
The main exception to this will be a Top Rated seller who is selling Charms but is "keyword spamming" by including the words "Links" and "London" in the title but without saying it actually is a LOL charm. Always read the title and description very carefully to check whether the seller is being straight forward or trying to mislead you.
The LOL hoops on the charms are oval. They are generally about the same size - 8-9mm long x 6-7mm wide (overall). They always have the LOL triple hoop logo on one side which can vary in size slightly and sometimes are heavily impressed and sometimes very light. A fake charm may have this logo also but often they are quite large or look very "rough".
The hallmark on LOL charms can be confusing. All silver LOL charms are made of sterling silver - that is 925 silver ( and may also include gold plating, rhodium plating or enamel).
Sterling silver charms that weigh less than 7.78 grams no longer have to be fully hallmarked. Most LOL charms will have "LL" in the hallmark along with the numbers "925". This can be impressed on the inside of the hoop or on the outside at the top, bottom or side of the hoop. However, recently LOL have been simply stamping "925" on the inside of the hoops which has made it more difficult for buyers on eBay to check this.
The Links charms always have a LOL "merchant number" and any genuine seller with a LOL receipt will be able to confirm this merchant number with you as it is on the LOL purchase invoice/receipt. They should also be able to tell you if it is a UK charm or perhaps one that has been sold abroad only and not in the UK. They should also be able to tell you if it was a web exclusive charm or if it was sold in mainline stores - perhaps with details of when and if it has been discontinued.
Any questions and I would be happy to help further if I am able.
You may also want to read my Guide on Links of London Packaging.