Wow, that is seriously upsetting. Any pet store with brains would put aside any sick animals and treat the poor things, not leave them out for sale like that. What a shameful store that is.
I would work on fattening her up gradually; I am not that familiar with abscesses in rats but it seems like something that is worth treating. Like the others said I would wait and see what Becky posts; I know she will have some input here.
Man people; I seriously am tired of these freakin' stores being so cruel to their animals. It seriously makes me want to punch someone! (and I am not violent!)