You've got some really great advice, but your best mark so far has been joining YT and learning what you can before so you can make the right purchase.
All I can say is: Forget the Classifieds!!!!
There are so many wonderful breeders here on YT who will give you the healthy, happy and well socialized new addition to your family.
...also, be prepared to possibly spending $1000
"IF" you find that perfect baby!
I thought I had 'researched' before I got Roxy and TJ (I hadn't stumbled upon YT yet). Anyway, Roxy was $800 and TJ was $700. They do not have AKC papers. I could have reg. them CKC (but for what reason???) Anyway, besides some 'minor' defects (TJ I believe has 'roach back' - his hair is curly and kinky - about 8 of his adult teeth never formed - Roxy is very short and the breeder neglected her teeth for 6 months, so they were very bad) and then the 'major' defects (Both have Luxating Patella's in Both rear legs - Roxy has treacha collapse - TJ had IBD) - They are 100% Happy, Spoiled and Well Loved Yorkies - Who when all is said and done and all LP surgeries are complete - They will be Our $10,000 Yorkies - And that's in ALL SERIOUSNESS
...so please do your research - find a reputable breeder - and be prepared to 'maybe' spending $1000 - But you'll have the peace of mind that your baby is health