Hi! I've been out of the pony-buying game for a little while so I'm a little rusty, but probably, assuming the ponies are in good condition with no marks or rubs or haircuts, it'd be like (in USD):
Sundance and Megan: $15
Baby Surprise: $15 to $20
Baby Lickity Split: $12 to $20
Molly: $10 (but possibly more because it's hard to find her so complete)
Sniffles and Snookums: $15 to $20 (probably on the higher end)
If you sold some of those accessories separately, they might bring in a little more money, but your time might be worth more. Megan's shoes, and Molly's shoes *especially* are hard to find. These sets themselves are fairly common, but the amount of accessories makes them appealing to accessory collectors.
Other users feel free to chime in with their price suggestions!