I understand your concerns as to the 'industry standard' on size to sell, we all have our own opinion of standards. HOWEVER, it isn't that the animal was unhealthy or not-feeding or 'ready to leave the nest'... it was only that you couldn't find the correct size food for the size you received right? So the size really isn't an issue since it was feeding, healthy, and ready to go in that sense... except that you 'misunderstood' her statement "around 2 inches" and expected it to be bigger; therefore not preemptively acquiring food prior to her arrival. If it were me (and I was concerned about size) I would have asked for a particular size baby and not used an ambiguous statement like "a smaller female". I also would have made a point to ask exactly what size food I'd need and be sure to have it the day (or two) prior to the animals arrival. If you'd checked for food locally the day prior, you would have known it was not a possibility, and would've been able to get bugs overnighted to arrive the same day she did. Not to berate you with the above statements, but maybe they are something to consider later... it's always good to be prepared in advance to getting an animal.
As for expecting them to send food, I don't think that is their responsibility either, they are responsible only for telling you what they need and from there, if you purchase the animal... they assume you checked for and have those things. For example, I know that I switch my baby snakes to prekilled usually before selling, it is easier for some folks to acquire since some stores only carry frozen rodents. However, if I sell a baby that is only eating live pinkies, I tell them they are live pinky eaters, but I don't expect to have to ship them live pinks to feed the snake after it arrives... if you catch my drift. Perhaps the seller could have had better word choice in their frustration with your arguement over food (I would have been frustrated as well, because I don't necessarily agree with your arguement and you are very steadfast in your opinions), but I wouldn't have held them responsible for me not being able to find the proper food locally... since I would have double checked earlier knowing such a small animal (around 2 inches) would be fragile and need to eat regularly.
I think the only thing the seller is guilty of doing (since they were upfront the whole time about size, age, and food type) is just poor word choice after you vented on them, the rest is a learning experience on your part. I don't think they are wrong for selling small animals they felt were ready to be sold, but I do think it is wise for folks who are less experienced with that age and size to know their limits or whether it be acquiring food for them or in experience.
Good luck with your critters, I hope she makes it!