I remember seeing this auction and your brag...these girls are adorable and its cute to see how Prima must have sold the babies. Yes, the ponies are regular HK releases but the way they are carded, who the are carded by not to mention I believe they are OOAK at the moment could do wonders for their value.
I am not even going to guess where they could price themselves you would get a much better estimate over in Nirvana - but if you are selling I would go with an auction and a set reserve. The problem with OOAK or previously unsold on an open market [where the auction is known to all collectors and the item properly marked as what they are] is that they could go much higher or even much lower than we expect!
Good luck whichever way you go!