Villagers sitting on the bench

Talk to Isabelle at the counter in the Town Hall and choose "problematic citizens." This will allow you to choose the neighbor, then give you some options. The options are:

"His/her language is rude." This resets their greeting and catchphrase.

"His/her clothes are wrong." This randomly changes their clothes.

"He/she has a naughty letter." I'm not sure what this option does. Deletes the letter I guess.

Also, when you put things up for sale at Re-Tail, your neighbors will occasionally come in to the shop when you're there. They will randomly select an item which will be announced by the alert sound and surprise lines over their head. You can influence them to purchase what they're looking at. If you have clothes in Re-Tail, that's another possible way for them to change their clothes.

Feng does not affect Re-tail. As Ben Brocka said they do always add an item every day. After the items have reached a pile of 5 items there are 3 days in which the items will remain. Afterwards everything resets and new items will begin to appear. It is just a coincidence that Feng seems to affect Re-tail.