It depends, your Flashcart will primarily be reading from the MicroSD card not writing. You really have no idea what the read speed will be until you get the card. If you know the country of manufacture (probably China), you can probably find a few speed tests from other people who have the same card.
Also the class speed is purely a minimum speed, they can often be much higher.
It's very risky buying SD cards from eBay though, many are often fakes.