The cards increase the overall rating but I believe the individual stats remain untouched. Remember, like half the cards just affect the behavior of the player, not his stats.
Speed Merchant doesn't make him faster, it just allows players to sprint and turn much faster. You'll be able to tell this immediately if you try sprinting with say Villa (speed merchant) and Torres (not speed merchant).
Marking just means the player is better at staying tight with his opponent when you manually assign an opponent player to be marked. I think this ability card replaced the old style where you could pick between 2 levels of intensity when marking players (tight and normal I believe).
I think its roughly the same for all the other S ability cards. They probably have a bonus effect in the game engine (maybe it affects the random factor) but again, I don't think they do stuff like dribble speed + 5, defence + 3 etc. The only one that theoretically does this is Super Sub as supposedly it allows a player to have a bigger impact when brought on late in the game. That might work like the form arrows which add/remove a certain percentage to each stat.