Another thought: what is Pinkie Pie? I'm not sure i'm that inventive but it's worth a thought.

I would try hand-washing just for fear of the washing machine being harsh, or I would suggest putting them in a delicates bag. Which pieces have stains?

I would personally steer away from StrawberryReef as it is no longer being updated and find that MyLittleWiki is a bit better of a resource for Nirvanas.
Rare, sought after, and expensive are like circles in a giant venn diagram that have lots of overlap but are not interchangeable. Rapunzel meets all three, but Mimic is not so rare huh? A lot of this can be picked up by just being in the community and learning as you go, which is what I suggest. I understand wanting to know which ponies are rare, but it's always a good idea to just collect which ponies appeal to you personally. Not every rare pony is the prettiest!
That said, the market is always in a bit of flux. For example, there was a time when the perfume puff ponies were not so sought after, and now? Red Roses is a bit up there! Things can certainly change. For average market value, I still hold the Bluerose guide in high regard as it is still being updated, but it is not perfect.
Also, just remembered the Wiki has a page about baits, including a list of ponies that should *not* be baited, which sounds like it should suit your needs of identifying ponies to not pass over.