Oh heck no...if that isn't a scam, my horse is purple with blue stripes!
My feelings, this probably is falling down like this: They bought your pony, with the intentions beforehand of trying to scam you out of your money. They made up bullcrap to try to sucker you into that. The pony may or may not have really colicked, the pony may have been put down or may be alive - if he did and was put down, it probably just fuels the fire making them push harder since they feel they have an "excuse" now. One thing is certain: You KNOW the age of the pony, and ponies don't use lose teeth that are otherwise healthy over a few days. So you KNOW they are lying about that, and obviously wanted money along with it, so you KNOW that part is a scam. Which adds weight to the fact that they are probably lying about the colic, in some elaborate scheme to, pardon the phrase, have their cake and eat it too - in other words, get the pony without losing their $$.
I would play along with their game to find out what is and is not truth. As was suggested by someone else, ask to see the vet bill. At least them you will know what happened. I assume they may have already buried the body, but I swear I would have asked to examine it to see if the pony did or did not have teeth, and get a vet to give you the professional opinion on the pony's age.
But honestly, don't give them a dime no matter what the outcome. In horse sales, it's buy at your own risk. If you waive a PPE, then you are taking a risk that the horse may have underlying health issues. And as was mentioned, it's unlikely colic could have been seen coming in the PPE anyway. Even if the pony WAS 30 and had no teeth, it was buyer's responsibility to check, check and double check (although I believe you are the honest one here!).
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers