The Retail feature was added in the 1.7.0 update.
It can sell up to 3 items at any time.
The retail facilities and their tasks
It automatically sells your items. A "base amount"(see formula below) is multiplied by the retail store count.
Customer support center
It increases your customer support rating. As a rule of thumb, make 1 customer support center every 5 retail stores, rounded up. (Example: 16 stores is 16/5=3.2 or 4 customer support centers)
Allows you to control the retail stores. You only need 1 of them, but the game allows more than 1.
Retail selling formula
Not much is known, but testing shows 2 rules.
When the price in store< price sellable to the NPC at (+0.00%)
The demand skyrockets. You can reach 5,000 cars/fac/hr when selling at 15,000(though it's not a great idea).
When the price in store > price sellable to the NPC at (+0.00%)
The demand is:
(demand at (+0.00%) at 1 store)/2^(log 1.25(price in store/ price in NPC selling))*retail stores.
In layman's terms, double the stores give double the output, but only a 25% increase gives half the output.