Protect Against Abusive Currency Exchange During Overpay

[joekiller]

Is your feature request related to a problem? Please describe.
People can offer a key for a 0.05 priced item and the bot will dump all the metal it has for the key even if it isn't selling metal for keys.

Describe the feature you'd like
I had a few ideas:
a. Create an option to deny trades where a key is not in the sale price of the item.
b. Create an option to limit the overpay of currency to a certain percentage of the sale price, say up to ~10% overpay of the value of the item, to help make change.

[zudsniper]

...
@joekiller

Those options seem like a good way to combat this. My bot is currently stuck with keys but no ref, I would assume for this reason. In regards to (b), the percentage should probably be configurable. Good ideas!