Sorry, I'm not trying to be rude, if that's how you feel. ^^;
Honestly, your values are your own and that's for you to decide. But if, say, a Forme Changer = 1x OMG so common = 500 AC, I think you could work your way up from there.
These would be about what I'd trade:
300-500 AC = OMGSC
500-1000 AC = Common
1500-5000 AC = Uncommon
5000-10000 = Rare
Make an offer = Very Rare/Old Rare/List
