I go by /~Owls values, but generally prefer doing item-for-item trades (especially for higher-value items), but if there's something I really want, I may offer GBCs/custom :)
I also have sale sections for GBCs at the very end of this list, meaning some items are 2 items = 1 GBC/4 items = 1 GBC (or item equivalent), as well as a FREE section (no limit per person, except if I don't have enough gift boxes).
BF GBCs x1
Styling Studio Supplies x2
Faerie Quest Fortune Cookie x5
NC Archives Fortune Cookie x1
May Fever Premium Mini Capsule x1
Styling Studio Prismatic Brush x3
Prismatic Mirage: Nostalgic Royalgirl Aisha x2
2 items for 1 Gift Box Capsule
4 items for 1 GBC
FREE! No limit (besides # of gift boxes I have)
Not actively looking for these at the moment, but if the opportunity arises... :)