It’s hard to pin point exactly what to sell because there’s a lot of shops and giveaways at Nook’s Cranny already, if that’s where you want to sell items.
I think that hard to find items like seasonal diys, Celeste diys, models, artwork, and stacks of material (especially seasonal) tend to do well. Perhaps selling NMT or IGB?
You just have to be patient because the game has been out for over a year, and it’s not as busy as it was months ago.
I don’t need anything atm, but what I buy often are: igb, materials, some unorderables. I also have to finish my clothes catalogue so there’s that but they are low on my priority list unless there is an event going on that involves ACNH.
unorderables for sure. right now im on nookazon awaiting a reply for a blossom lantern. ive also bought every *and have them given to me as well thankfully* pocketcamp item available and im also searching for the festivale parasol and other hard to get items.