Shopping is a mandatory part of being a trainer, or at the least any other carer of pokémon - after all, potions can be used to heal just about anything even if you weren't one to seek battles, from bumps and scrapes to residue confusion from a Flatter and burns, not to mention the use of Escape Ropes and repels if you were a traveler.
Mew knows trainers are very well-versed in just how useful repels are.
So it's not like it's anything unusual, Green mooching by the PokéMart and a shopping basket looped over an arm crammed to the brim with Potions and PokéBalls. He skims over, disinterested, the array of PokéDolls, vaguely picks up a different type of PokéBall called a Heal Ball to examine, puts it down, reaches for--
And, oh, look at that, hands stretch simultaneously and bump on the way to the last Full Restore on the shelf.
Green scowls immediately on impulse, pulling away by proxy of stranger etiquette but not backing down.
"Hey, I got here first."
