Nifty Knitting SP SimGuru/Maxis-Sourced Info

Plopsy

SHOP ONLINE WITH PLOPSY
The more you knit, the more you can earn on Plopsy, the new online crafting marketplace. List or buy everything from socks to sweaters to paintings and woodworking projects. For a small fee, you can find the perfect buyer to make the most profit, then send it off via your mailbox. - EA

As a bonus, Plopsy offers a higher payout than selling things the old-fashioned way.

Now, if your Sim is more of a shopper than a crafter, you can also use Plopsy to buy unique goods! Plopsy rotates in random crafted items throughout the day, so all your Sims have a chance to get some cool stuff (including the new knitted clothing!). - Community Blog

You can keep track of your listed items in your inventory, where you will see the current duration of the item and if there are any interested buyers.

If someone is interested in buying your item, you’ll be notified with a message. If you agree to the buyer’s offer, you can use the Ship on Plopsy action to ship the object in the mail. You’ll be paid immediately and get a thank you message from the buyer when they receive your package. Alternatively, you can ignore a buyer’s offer and relist the object for sale again.

... We plan for Plopsy to be available for things like paintings, woodworking statues, potions, knitted items, flower arrangements, and more. - Deep Dive

https://twitter.com/SimGuruNinja/status/1283115042125168640
so, only base game items? No:
https://twitter.com/SimGuruNinja/status/1283123220330119169

Livestream info:
https://twitter.com/platinumsimmers/status/1286013509860458497
https://twitter.com/platinumsimmers/status/1286005071658881024
https://twitter.com/platinumsimmers/status/1286004568577515520
https://twitter.com/SimGuruNinja/status/1286024549650243585