Disclaimer: I don't own Harry Potter or any of its contents, and I'm not making any money off this.
Tom understands Muggle money. He has every ability to get a hold of some. And, even if he still didn't want to do that, he could probably just Imperius the employees around the door to let him walk right out with a handful of not-paid-for items.
Instead, he blows up the side of the building, storms into the pet isle, spends ten minutes deciding between rubber balls and then Disapparates before any Aurors show up. He doesn't leave a trace of evidence, as usual, and the only indication that anything out of the ordinary has happened is the smirk on his face, only available directly after a good murder.
He climbs the steps of his new home—a temporary place taken over from some undeserving Muggle—with a large, grey ball tucked under his arm. He's dressed in simple trousers a button-up shirt, so the Muggles will have a particularly difficult time being able to give any sort of accurate report. With his thick dark hair and his slender physique, he looks just like anyone else, really. With an extra facial glamour, he really could be anyone.
As soon as he walks into the living room, Nagini rises up to greet him. Tom smiles fondly and tosses the grey ball towards her—her long tail shoots out to swirl it around the floor. He hisses, "Hopefully you won't eat this one."
Nagini returns curiously, "Why would you make a ball out of rabbits if you didn't want me to eat it?"
"It was to play with," he chides. Her golden eyes twinkle; she knows he isn't really upset with her. He never is. He sits down in the large armchair across from the fire, propping up his legs on the coffee table. The glow of the flames dances over her magnificent body, and she uncoils as she slithers towards him, picking up the ball in her mouth.
It slips out, however, as her fangs make it rather awkward to hold it without puncturing the top and bottom. Tom laughs as she nudges it towards him, and she looks up with an irritated sort of huffing expression. "I'm not a dog."
"It's not for fetch," he chuckles. "I figured we could have some sort of game tossing it back and forth, seeing as how you don't seem to be very fond of Gobbstones..."
Nagini's forked tongue darts out of her mouth: her way of smiling.
It's a bit challenging to pick up without any arms, but she manages to hold the ball firmly under her chin and wriggle her tail beneath it, until she's successfully lifting it into the air. Then she uses her tail to hit it towards him like a bat, and he catches it right above his lap. He waits for her to slink backwards a bit and rear up before he tosses it over. She catches it in her tail again, swiveling through the air to keep it balanced. She tosses it back, and they play for a little while, until Mulciber comes in covered in blood to inform them he's messed up.
That's nothing new, and Tom holds onto the ball while Nagini 'plays' the way she prefers to.