This buys one less potion in Petalburg, which is 99% of the times fine. The timesave that skipping one potion means is just too good.

Petalburg Shopping
Buy: 3 Potions, 7 Repels.

Rustboro Shopping
Sell: 2 Great Balls, Rock Tomb
Buy: 3 Super Potions, 3 X Speeds, 3 X Attacks

Fallarbor Shopping: 
Sell: Nugget, Bulk Up, Steel Wing
Buy: 3 Super Potions, 11 Super Repels, everything else is the same.

The main timesaves are not selling Poké Balls (also good to stall against Flannery) and buying all X Speeds and Repels in one go.

GOOD																BAD
Instant Scrolling on Repels (from 1 to 7)							Awkward X Speed on Archie
Buying Repels in one go (around 4 seconds of timesave)				Slightly worse scrolling (not even a second of timeloss)
Buying X Speeds in one go (same thing)								One less Potion
Not selling Poké Balls (3 seconds)
More stalling options on Flannery

If you don't have to X Speed on Wattson you can just use the X Speed to stall on Flannery.
If you have 0 Potions after Roxanne you can just sell your Poké Balls and buy another one in Rustboro. Basically evens out with the old shopping.
IMPORTANT: If you fight Bug Catcher Jose, buy 2 Super Potions in Fallarbor and then 3 Energy Powders in Lavaridge.