Few comments...
The archaic [trade] sign format:
-1.7.2 items and newer are not in the [trade] signage code so will not work. (it its not on /lookup then it likely wont work) Unsure of the version? This can help..
https://minecraftitemids.com/versions So incompatible items include but not limited to elytra, shulkers, end cities materials, banners, fireworks, enchanted books, etc-If for some reason the name does not fit on the sign, numerical item IDs work too. (cake: 354, tnt: 46, etc)
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/lookup <name> can be used to look up names usable on trade signs
example: You wanna sell carpet?
/lookup carpet shows all the carpet names with the colours
-To sell enchanted tools you just put <# material tool> or "1 diamond pick" (this would require providing a sign telling what the picks in the chest is enchanted with.)
** +Ideally this could be used as a sort of tool mystery box. ie. Pay a diamond, get a random tool if you don't say what is in the chest.
-EVERY individual chest will require its own [private] sign. [trade] signs do not seem to work with daisy chain locking (this is where you can lock multiple adjacent chests with one sign)
**-[trade] signs are also incompatible with trapped chests.
**Regarding payment.... Is the IB per [trade] sign. or literally EVERY sign?
So every trade sign would effectively cost minimum 2 IB per month(one for private sign, one for private sign).
10 trades would be minimum 20 IB (10 private signs, 10 trade signs) +signs detailing enchantments
15 trades would be minimum 30 IB (15 private signs, 15 trade signs) +signs detailing enchantments
and so on... (I think most players know math lol)