First, ask if it is really a problem. Are the rest of the players bored watching a one-man show, or are they amused and having fun watching this part?
If it really is a problem....
Call for a Roll
Shopping shouldn't take all session, and if the player has had a bit of the spotlight already that session, then there is no problem with saying,
DM: Alright, we need to move things along. You're trying to persuade the shop owner to give you a discount on what? What is the end result you're hoping for?
Warlock: The stakes, the wooden box and few bottles of potions.
DM: Great. Roll for persuation, with advantage.
DM: You talk at length and convice him to give you a 20 gp discount. Do you take it or leave it?
Don't always go this route, but sometimes you just have to move on.
While X, what are you doing?
Purposefully shift attention to the other players. It allows him to have his moment, but not loose
Warlock: Your stunning array of woodcraft is beautifully carved. How much would it take to buy one of these fine pieces?
DM: Okay, so the warlock is going to try sweet talk a deal with the shopkeeper. While he's doing that, what are you doing, Fighter?
You'll see Matt Mercer of Geek and Sundry's Critical Roll use this a lot when the party is shopping
NPC Shift Spotlight
Or even have the NPC make the shift.
Warlock: Your stunning array of woodcraft is beautifully carved. How much would it take...
DM[as NPC]: Yes, yes, sell your flattery elsewhere silver tounged worm. [to Dwarf] Do you hail from the kingdom under the mountain, Yes? So far from home. What dwarven work treasures do you have to trade?
Or if you have a player with high passive perception, pass that player a note while they're talking about something they notice, while the Warlock tries his thing.
You notice blood on the hem of the store keeper's tunic, and the shopkeeper is trying to hide it by holding his cloak close around him. As you watch him, something else seems off. Is his accent different than the last time you spoke with him? Where his eyes always that shade of gold? Something is just... off.
They can then choose what to do with the information. Maybe they let the warlock keep going to see what happens, but now they get to enjoy an aspect of it the Warlock doesn't know about. Maybe they will interrupt the Warlock, and try to get the party out of there. Maybe they will try to confront the shopkeep in the middle of the Warlock's corruption/manipulation/persuasion tactics. How fun for all involved if it played out like:
Warlock: Your stunning array of [DM slips not to Rogue] woodcraft is beautifully carved. How much would it take to buy one of these fine pieces? I'm sure I could pay a fair price of...
Rogue: I stab the shopkeeper while he's distracted.
DM: Roll for attack.
Warlock: Wait! What?