Because if it was from shells, why wait until they have so much stats before selling them all at the same time?
Because she has a number of toons that can run gobs, and that was the day when they all reached 300 shells? Or she had the shells on alts already and this was the day when she had some time to actually spend them? Or she kept the stats for herself but decided she needed money now (to buy Tyrants, say), so she is trying to sell a few immediately?
Really, no point inferring things based on one-time one-toon incident, imho.