As far as I know, CardMarket is the cheapest and safest way to buy collectible cards in EU. I used it back then when I played Yugioh and never had a problem.

The alternative is to trade/buy in person at card shops and with other players. In that case, hone your bartering skills.

I would avoid EBay and other similar sites, they usually rip you off.

Cardtrader / Cardmarket.

First one aggregate all the items in their warehouse and you can send one big package at the end. But just bear in mind that the waiting time till the last items actually arrive to their warehouse can be really slow (it's probably on the post). You don't pay for to warehouse delivery if you apply certain filters, but cards there cost bit more on avg than cardmarket, plus they charge fees.

I used it a lot, but now I stick with market.