Selling keys on buy only

[Nevskjij]

Description
When autokeys is disabled and keys are set to buy only its possible to buy them with keys on buy only.

To Reproduce

Steps to reproduce the behavior:

1. Check the key rate.
2. Send a manual trade where you buy an item and use key as change. Make sure the offer is 1:1 with the key rate.
3. Enjoy the key you just stole.

Expected behavior
Decline the trade or make a new offer with only metal.

[Yahiamice]

this happened to me yesterday & my bot was on buy only & had keys at higher than bp prices just so no one would send a trade offer for 150 ref. little did i know someone abused that to buy everything i had listed under a key in ref for 50% off