Beside "The Winning Ticket" episode where bunch of customers showed up for the raffle, I could only count 7 instances in the entire series where customers showing up in Sanford&Son store to buy something.

1. Mortician showing up to buy coffins
2. Antique shop owner looking at Revolutionary War rifle.
3. Jonathan Harris (Lost in Space) show up to buy a commode.
4. Customer is buying a lamp which Julio's Nephew is using for studying.
5. In the "The Merger" episode, Smitty and Hoppy buying hubcaps.
6. Customer show up buying Fred's war medals in "Sergeant Gork" episode.
7. Two customers is seen in the "Aunt Esther Meets Her Son" episode where they are looking at bugle.

I feel like I am missing another one where Fred sold an item to a customer, and Lamont told Fred he like the way he handle the customer. Don't know, maybe I am imagining it :D

They had a big order for standalone bathtubs in the Gus episode.

They had a big order for standalone bathtubs in the Gus episode.

It escape me, but which episode was that?

Nope. I was wrong. Lamont wanted to throw out the tub in that episode called "A Guest in the Yard".

It was "Blood is Thicker Than Junk" with the bathtubs, but Fred was buying them. Fred sent his new "dummy", Norman, with a check to pick up some bathtubs from The Supreme Salvage Company, but the guy took the money and bought a stuffed bear instead!