Im going to build off of Aplauses' idea (so don't kill me if it sounds similar).
What if when we traded with people we could make an "loan" option for the trade. This would be a safe way to do loaning and to pay off equipment trades incrementally. Here's how it would work (sorry for the odd variables, A and B are just identifiers, X is a number): Knight A wants Item A from Knight B, but doesn't have Crowns X to pay for it. Knight A chooses the "loan" option from the trade menu. Knights A and B then agree upon a price. Knights A and B enter their sides of the trade into the loan option and both Knights confirm the trade. (Seems similar to what we already have? Just wait.) Once both Knights have confirmed, Knight A gets Item A and Knight B gets Crowns X, HOWEVER the Crowns X that Knight B receives are NOT those of Knight A. These are Crowns that are loaned to Knight A from the system through the "loan" option. This means that Knight A owes the Crowns X and some extra (it's interest, but it's a set amount extra as opposed to an increasing amount). The way that Knight A pays back the loan is that whenever Knight A gets ANYTHING that can count towards paying the loan off (with the exception of mist energy because that can't be traded) it is AUTOMATICALLY sent to the "loan" account and the amount given is taken off. This system could work with any type of payment (CR, CE, mats, gear).
This also can be implemented with a third Knight as the loaner, instead of the system. That would work like this: Knight A wants Item A from Knight B, but doesn't have Crowns X to pay for it. Knight A chooses the "loan" option from the trade menu. Knights A and B then agree upon a price. Knight A then finds a Knight to loan price of Item A and sends a "loan request" to Knight C. Knights B and C enter their sides of the trade into the loan option and both Knights confirm the trade. Once both Knights have confirmed, Knight A gets Item A, Knight B gets Crowns X, and Knight C gets a "loan account". This loan account works exactly like that of the one in the previous system, except that the items/crowns/energy are sent to Knight C.
POSSIBLE ISSUES
Knight A exploits Loan to get "free" gear: Solve by implementing a "foreclosure" method. If the Knight stops playing for a set period of time (agreed upon by Knights A and C if the 3-Knight system is used), then the item involved with the Loan is "foreclosed" and sent to Knight C (if the 3-Knight system is used) or it is destroyed (if the 2-Knight system is used). Knights will not be allowed to owe more than one Loan at a time.
Knight C exploits Loan to get a steady source of income: Solve by not allowing Knights to loan to more than one Knight at a time.
Knight A is an alt for Knight D and sends Item A to Knight D so that Knight D gets Item A, but Knight A has the Loan: Loaned items are untradeable, undroppable and bound while the Loan is still in effect.
(Just a final thought, this could give way to a Knight Credit Score, like how well/quickly Knights pay off Loans, making them more or less reliable to potential Loaners (Knight C))
TL;DR?
It's basically like a mortgage system through the trade function.
EDIT:
I like Autofire's idea of making the items unusable as opposed to destroying them. Also, maybe it could become unlockable with tiers.
Asukalan: Everything used in the system (other than the crowns given to the selling knight) stays in the system. The fix for your situation would be to use the 3-Knight system.
Destroyed? I would just think it would become unusable, or the giving knight would be given it back. If it's destroyed, some are going to be raging over the fact that that brand new FoV you just got was destroyed. The system seems a little to complicated for new knights, too, and some might be scammed by it.