Heres just a few pointers.
Someone working at petsmart which I'm guessing is a pet shop, will have no medical diagnostic training, they won't have learnt anything about medical issues etc. They're simply taught how to house and handle the animals they sell. Only a vet should diagnose and recommend a good treatment.
Secondly any creams shampoo's bought from a shop can't help your pig, anything sold in a shop, will be limited to how much of certain chemicals etc. it can have meaning that 99.9% of the time it's ineffective and will leave you spending money on something that doesn't work and still having to go to the vet after. This is the same with flea treatments that are bought in the shop they literally have no effect as they're just not strong enough.
I know sometimes it can be tight with money taking a pig to the vet, i've experienced it before, but it's whats best for your animal, and i'd much rather go without buying something i wanted or without going somewhere than have to watch any of my pets suffer.