Hi mate, for anyone to be able to tell if your 2 Sand Monitors are Flavirufus or not, you will need to share a photo showing the whole body and tail, so we can tell what sub-specie they are based on their markings.
Your photo does not give much detail of their markings at all.
You also want to know if the yellow one is less common in captivity or not it's hard to say how many of that colour phase are in captivity, and for a price of your yellow Sand Monitor, you could sell it for roughly $275.00 to $350.00.
as long as it's in 100% good health and not missing any toes or claws or tail and as no injuries and feeding well and good to handle.
The price depend on supply and demand and how many are available in the pet trade at the time you want to sell them.