Show quality is subjective at best. Show in what? Halter? Color? Obstacle? Driving? Those are all 'showing designations. Most assume it's halter, but that isn't necessarily the case.
Then again I've sold horses that I didn't think alot of, but have done well at regional/nationals. Proper training, fitting and knowing how to show can make a difference as long as the conformation is there.
As for people selling on CL's, in this economy I've seen many people use it, and it's a free ad to your local area. The prices from breeders are higher than the average grade CL's Mini, but I've not asked how well people have done selling through that venue.
Where you find your Minis isn't the issue IMHO. The issue is - is the horse healthy, sound, good conformation (or better), paperwork in order, etc., and is the horse what the seller is representing it to be, before you put your $$ down. Contact the seller, if it has a registered name, contact the breeder and/or the registries.
I sold a Mini a couple years ago that was resold, then hit CL's. Someone alerted me to that, I contacted the person selling and they had been grossly misinformed by the person they'd bought from. He was sold on a gelding contract (the buyer disappeared on me), but was represented as a breeding stallion, etc. (lots of misinformation), Fortunately the new owner believed me and changed his ad to reflect what I told her, he was gelded and ended up in a good place.