What is the reason for not storing the actual price in the market? Why not keep things simple and ignore flipped markets?
Then you have to decide which one is "natural" and which one is "flipped".
bytemaster says the "natural" market is BTSX priced in BitUSD, because US dollars are a relatively stable and familiar unit of value.
I say the "natural" market is BitUSD priced in BTSX, because BTSX is like "real money" (a limited-issue token) and BitUSD is some kind of derivative contract.
Letting each investor have their own preferences is a compromise between these viewpoints.