After buying Nifty Knitting, and playing around with plopsy a bit, I have discovered that IMO plopsy is a very convenient feature that is easy to like for its household marketing scheme, while at the same time, being about as entertaining as a glass of water.
Plopsy is a copy/paste script that we've already seen pretty much since day one (6 years ago) that allows you to browse the store-like listings.... no differently than buying seeds, or shop items.... that allows you zero user options other than "buy" and "quantity" of items.
Add to that the selling side of plopsy being limited to rabbit-hole sales. You click on item, choose "sell on plopsy", and that's it. Item disappears from house, and shows up in sims' inventory.... which you can then see plopsy info by hovering mouse over item.
As for browsing plopsy to see your listings.... that option doesn't seem to exist. You can browse plopsy to see what items you can buy, but not to see what items you are selling. To keep track of the items you have listed on plopsy you will need to visit the sims inventory.
And yea... that's plopsy.