After watching videos and reading comments.... (Don't have NK yet)
Basically, plopsy seems like a copy/paste of the store-like script that pops up when you want to buy stuff like seeds, medicine, or items from the ROM shops.... that doesn't really do anything extra except give you the option to buy pretty much most things offered by all the packs as a whole..... rather than be limited to one pack such as the ROM store popup.... or the buy seeds popup.
I'm not sure if its fair to consider it like ebay or etsy, or more so like an all-in-one mall script. I think the only thing that makes plopsy seem more like an online marketing tool is that you don't need to travel to a shop to use it... such as ROM shops.
Overall, plopsy seems great when considering the idea of it, being that it opens the door to buying and selling at least most items in the game. I like the marketing value it adds to the sims households.......
........ However, when it comes to "using" it. It seems like something that would be completely boring. It doesn't have it own look and feel separating it from the obvious script it was copied from, it doesn't come with any notable features (can't create profiles, set prices, etc), there's no need to manage the sales by mailing the sold stuff, and on the selling end of plopsy, it seems we don't even get the store-like popup.
So far, again, based on videos and comments shared by others, plopsy seems more like a semi-rabbit hole mall of a buy/sell feature for sims pack items.
Please feel free to correct me if i'm wrong.
Edit:
..... With the economy tweak leaving my rent for a mansion size house at being just 500 or so simoleans a week, my 4 sim household really doesn't need more money. So when it comes to plopsy, I was more interested in the "entertainment" value side of the feature, not so much the "economic" side.
I was hoping plopsy was fun to use, not just be economically convenient. It's ok if I only get 1200 simoleans for a painting by clicking/selling as usual rather than 4000 using plopsy..... because truth be told... my sims don't even need the 1200.
Edit:
.... just bought pack so i'll see for myself what plopsy is about.