I think an honest seller on Craigslist would be willing to meet you at an appraiser or jeweler (that you choose) to have the ring evaluated. They should also hopefully still have the certificates ( GIA, etc) that they got when they purchased the ring.
If that seems like too much work, a lot of people on these forums have had good luck going through Costco/BJ's/Sam's Club (seriously). Or, online reputable online retailers, like Blue Nile or James Allen. Blue Nile has a lot of selection, you can see the stones before you settle on them, and their pricing is lower than brick and mortar places.
Also, double check that she wants a diamond. People are going lots of different directions these days, moissanite, sapphire, ruby, emerald, lab made diamond. All of those options are unique and could be cheaper.
Good luck and congrats!