For the early game, there's no practical use. The sex toys unlock some masturbation scenes, but that's it. Save your money.
Mid-game, after the fight at the Diamond Mountains, Shopkeep switches to selling items that give stat bonuses. Nothing huge, but they're useful. I recommend prioritizing the ones that gives Skill (the spine and vertebra) over Power (the claw and talon), since Skill is harder to come by and the spine also unlocks a scene later on if you buy another thing Shopkeep sells further into the game. The energy drink can probably be skipped, its only purpose is to help you pass one specific Power check. If you've been training whenever possible and gotten other bonus Power options, you shouldn't need it.
Whatever you buy, you're going to want to save $32000. Once you get access to the dream world, you'll find that Shopkeep has a shop there, too. Make sure to visit his shop before you do any other optional things in the dream world, because for $12000 he sells an item that unlocks a pretty interesting scene, that gets even more interesting if you've also bought the spine from earlier like I said. The other $20000 is for a mystery box that his dream world shop will sell later on, we don't know what that does yet but it's probably worth picking up.
Finally, in this very latest update, Shopkeep starts accepting Power points as a currency. The items he sells are pretty hefty costs. He'll exchange Power for money, giving you $50000 in exchange for 10 Power, which suggests that money might be of use later on. He sells two items that will raise or lower your Corruption by thirty points for 20 Power. And he sells an item that raises Skill by 3 in exchange for 30 Power.
Shopkeep also implies that we may need every bit of Power we have soon, though, since he is willing to extend a line of credit for his store. He'll give you one of those items for free now, so you'll be at full strength for upcoming events. but add an additional 5 Power charge when he eventually comes to collect. I say make a save before you go for any of the Power deals, just in case next update there's a Power check you won't pass if you bought anything.