I don't actually sell any of my items on the flea market. I just go in to sell to Reese. Because my flea market is almost always empty, MOST of the time there is a new item being sold by a villager. I just use this item as one more item I have access to each day. As long as the flea market is not full very often the items continue to pile up until the store is full (I would assume). Which brings me to my question.
I have only ever made it to 5 items 'piling up'. I had already owned all of them so I had left them there. But after 5 days new items stopped arriving. I don't remember the particular amount of days because I just figured the limit was 5. So I bought 1 item hoping that another would get filled in the next day but nothing did. So then I bought the last 4 to hopefully reset whatever the problem was and for a few more days still nothing showed up until one day there was another item finally. After this point they started arriving every day again. I don't know what I did differently.
Now the past few days I have a single item in the shop that I am not interested in and 2 days one more item showed up (that I bought leaving only the 1) but the past 2 days no new items have arrived again.
I would say it is just random but it was extremely consistent the first few weeks and even when it piled up to 5 they were 1 per day until they stopped. After they started up again they were also consistent up until now when they have stopped.
My Feng Shui is rather terrible as I trip more often than I would like and my Money Rock has been rather paulty lately. Nothing I have found so far has suggested Feng Shui affects Re-tail but it's the only thing I can think of.