We define a playtest card as a card (typically a basic land) marked with the name of another Magic card for the purpose of playtesting. Playtest cards aren't trying to be reproductions of real Magic cards; they don't have official art and they wouldn't pass as the real thing even under the most cursory glance.
It is common practice for players to use playtest cards to test out new deck ideas before building out a deck for real and bringing it to a sanctioned tournament. Wizards of the Coast has no desire to police playtest cards made for personal, non-commercial use, even if that usage takes place in a store.