It is illegal to sell fake Pokemon cards, so if you are a shop, always check that the cards you buy are genuine.
In this day and age with increasingly complex technology, it is becoming more and more difficult to determine what is genuine and what is not. If you are a retail store or another form of reseller and you are caught selling counterfeit Pokémon cards, the penalties can be quite severe.
Another reason not to sell these cards, is that the quality of them are more often than not poor. This means that cards once played with, start to separate from the from the back of the card which will inevitably cause customer dissatisfaction. Also such cards are usually poorly printed and are not as clear as the genuine article.
A good way to check if your cards are genuine or not, is to ask the question outright. If you receive an email from a supplier claiming that the cards are genuine, you may have a partial defence if caught selling counterfeit goods. Most people selling fake cards will either not respond or would not answer the question. Most of those selling genuine cards will answer the question “yes”.
Once you have purchased your cards, if you are purchasing a booster box for example, the plastic lining containing the cards will have the Pokémon logo printed on it. Also it is easily printable, most counterfeiters do not bother with this detail. If you are buying in single card packs, the package themselves will feel flimsy, and if you open them up, most will not have online code cards as they will not work, and the cards will normally be lighter stock and all genuine Pokémon cards have a black strip between the front and the back these just visible to the naked eye.
Some very glaringly obvious dummy cards, are the ones with unusually high hit points, or poor quality printing.
Another giveaway is usually that the card packs are unusually low in price, with card packs at unusually low prices.
Although you cannot fully protect yourselves from counterfeit producers trying to make profit from genuine companies, if you follow some simple rules you should be able protect yourselves somewhat.
Good luck playing with your, genuine Pokémon cards.