TCGPlayer vs Cardmarket prices
Use TCGPlayer if you buy or sell in the United States and Cardmarket if you are in Europe. They are separate marketplaces with separate supply, so the same card routinely carries two different prices, and neither is more correct than the other.
Why the two prices differ
Print runs are distributed regionally. A set that shipped in volume across Europe leaves Cardmarket well supplied and prices soft, while the same cards stay scarcer in the US and hold a higher figure on TCGPlayer. Currency movement then widens or narrows the gap on its own.
Shipping and import costs keep the two markets from equalising. Moving a five dollar card across the Atlantic costs more than the card, so the arbitrage that would normally close the gap does not happen at the low end.
Which number to use
Price against the market you will actually trade in. Valuing a European collection at TCGPlayer prices produces a total you cannot realise locally without paying international shipping and waiting weeks.
For expensive cards the gap can justify the effort of selling abroad. Below roughly twenty dollars, fees and postage will eat the difference.
Questions
Which is better for selling?
Sell where you live. Both charge seller fees, and cross-border postage plus customs usually cancels out whatever price advantage the other market showed.
Why is a card cheaper on Cardmarket?
Most often because European print runs were larger, or because the exchange rate is working in that direction on the day you look.