I can't come up with a full kit for this because I am dumb, but I like the idea enough to at least throw it out there.
Also, I'm silver 4 in solo ranked and have little experience in game design, so make of that what you will.
Passive - Set up Shop
The Dark Merchant - let's call him 'Bob' - selects a turret and sets up a bazaar around it. An allied champion near the bazaar can buy items as if they were at the shop but does not heal, gain homeguard or other similar effects. The bazaar grants a 10% return in gold on bought items. You may only have one bazaar up at a time.
Bob can also set up a bazaar around the ruins of a turret, friend or foe. (think Azir's passive) If he does so, the bazaar has 200 health and grants a small amount of vision, but the enemy can use the shop as well.
Bob may only set up one bazaar at a time. Setting up a new one destroys the old.
The idea is very simple: Keep champions in lane for as long as possible.
The ideal situation I see this used in is that Bob goes bot lane with the ADC. Said ADC farms decently, but more importantly retains a lot of health and doesn't get kills. They need to go back to the shop for power, but they have no quick way to return once they do, potentially giving the enemy free farm. You have them shop at your store, you gain some gold, they gain their items, problem solved. A second possibility would be to set up the shop in the mid lane, where your mid and jungle both have quick and easy access to it at all times. Your adc doesn't have very quick access to the shop anymore, but your midlaner does, and your jungler can get to it midclear. This gives you the advantage of gaining 10% of the spent gold of two champions instead of one (The jungler can also go to the bazaar at bot lane after clearing the bottom half of their jungle in the typical scenario, but mid gives them better access)
You can also give it to top lane if your strategy requires you win top lane... i mean this seems incredibly ineffective to me, but w/e, it's your strategy
The advantage this kind of thing would grant should be fairly obvious: you have a second shop which means you can stay in lane longer, you don't need to back unless your health gets low, and your power increases faster than your opponents. Bob would be strongest in the laning phase, when you have good reason to stay near your tower and your items are not complete as of yet. As towers go down he has to either choose between making his bazaars accessible to the enemy, or keeping them further from the front lines and thus less useful. The 10% gain is large enough to grant him an increase in gold income, but not so massive that it can be exploited. When his allies and enemies get their full builds the shop becomes near-useless for much beyond vision, but by that point his team's in their late game and it's up to them (and the rest of Bob's abilities)
I had an idea for the guy's fluff for a while of this dark merchant character that would trade you what you wanted in exchange for some horrible cost. However, he's very up front about it and sees the deal as perfectly fair so long as you take it willingly. It's a huge cost but you're gonna take it, because whatever he's giving you is worth more to you.
Kinda like Tahm Kench, with less magical journeys and more capitalism. But the major difference I see is that the Dark Merchant sees himself as a good guy, helping the only way he can: through business both mundane and magical. He's not out to make you miserable, he's just there to earn some coin.
This scenario doesn't actually make sense for League, but it's what sold me on the idea, so here it is anyway: A mother is trying to lift a crashed car to save her daughter, who's trapped beneath. As she's doing so, the Dark Merchant offers the strength to lift the car, but only if it's the last thing she ever lifts. She agrees because hey, her kid is dying, and he grants her the strength. She thanks him and asks how she can repay him. He holds out his hand and answers, "my payment." The DM (heh) gets all the strength the woman had before the boost, and she's crippled for life. Not exactly a fair trade, perhaps, since she only got the strength for a minute or so, but she probably won't complain; her daughter is alive.
"The cost is high, the payout is higher."