I guess I am asking is there any chance selling it on my own would get much more than the 30% loss I would be taking by selling it back to the jeweler?
If the original seller is willing to buy back at 70%, that sounds like a pretty good deal. Unless you have an OEC or some other stone that isn't readily available, people shopping loupetroop, diamond bistro, etc. will be looking for at least a 25% discount, and I think sellers there often end up getting even less than 75%. Plus with a consumer to consumer sale, you'd have to deal with shipping, the uncertainties inherent in most internet sales, etc.
If you have the actual AGS cert you might try PS's "sell your diamond" feature and see what offers you get. You can also try it without a cert, but you'll likely get lower offers.
If you don't have a deadline to participate in the buy back program then you could try selling it first on your own. People are always in the seem to be looking for ags stones so might be worth a try. You can always fall back on the buy back program if needed. You can post in preloved and add pics, info and links to a listing at LT or DB. Just some thoughts......