FWIW...my 3 year olds get a large adult mouse every 7 days.
IMO(and I am by no means an expert), those snakes are seriously underfed. While it is true that in the wild, snakes may very well not eat as often as every 10 days, it also very rare to find a wild mouse nest with only 1 fuzzy or 1 pinky. I imagine that a wild 3 year old corn that came across a mouse nest would eat every pinky or fuzzy in there, and possibly the adults, too. I also imagine that a wild corn would eat lizards, smaller snakes, small fish, and probably even insects, if necessary, and I seriously doubt that this seller is "supplementing" her snake's very small diet with any of those items. Fuzzies and pinkies simply do not contain enough nutritional density to allow a corn of those ages to grow and thrive. And quite simply...snakes held in captivity should be doing more than simply surviving...they should be thriving, and living as healthy and complete a life as possible. That is our responsibility to them as their keepers.
I seriously doubt that there are too many wild corns that thrive on 1 fuzzy every 10 days, or even worse...less food, without serious physical limitations being in place from a lack of nutrition.