Stall causes a Pokémon with this Ability to strike last within its priority bracket. (Bulbapedia)
There are several levels of priority - most moves are priority 0, Sucker Punch is +1.
If Sableye uses Sucker Punch, it will go before any move with priority 0 or less. If the opponent also uses a move with +1 priority (e.g. Bullet Punch) then Sableye will always go last whether or not it is faster.