Resolve guild trade-in exploit

[ibm2431]

Usual: "Servers should already have this. If not, contact zach2good. Full explanation later."

Anyone cherry-picking: You'll need our previous NPC trade exploit fixes which set/use reserves.

I affirm:

• [x] that I agree to Project Topaz's Limited Contributor License Agreement, as written on this date
• [x] that I've tested my code since the last commit in the PR, and will test after any later commits