Personally it always was my understanding as well that you can sell to or buy from the shop owned by another character of yours if:

Anyone shopping there gets paid/asked the same price for items

Anyone has a chance of shopping for those items (so you're not buying the one single item of a certain type you just put for sale)
To do otherwise is, in my opinion, taking the 'no muling' rule to an extreme that is unlogical.
Especially if it might quite well be that *other* characters working in the shop might use the items you sold to them with an alt.
It certainly seems to be no longer a Common Sense applying to me.