In the previous AC games once we sold an item we were able to order it back from Nook. But are we not able to do it in this one? I sold an item to Reese and then I went to Tommy's/Timmy's to see if I can order it back, but there was no option >.<
Just like in previous games some items can't be reordered if they are rare or have a different collection method. But all normal items should be reorderable regardless of what you do with them. Some examples of non-reorderable items include villagers pictures, themed furniture (like spooky furniture), fossils, gyroids, fortune cookie items and things won in competitions like the bug and fish themed furniture.
Also the catalogue doesn't become available until the Nook store is upgraded to T and T Mart.
The one's you need to be most wary of selling are things that can't be gotten again on that character such as the rewards from the post office for saving up bells.