I don't know the logic of this custom, but I learned from my parents this morning that one shouldn't sell stuff on a Friday. The issue cropped up after we had told a vehicle broker to pick up our family's 18-year old two-wheeler in the evening.

Father had to then go to his shop and request him to bring the buyer on Saturday.

It would be interesting the know if this custom has any basis in our shastras. Jotting it down anyway.