It's actually simple.
Because you have to redeem it first on the website, then trade, you can simply say this to your buyer.
You: I am selling the mount on multiple realms for the best offer, if you don't wish to pay a deposit for the mount, then I won't redeem it on this server.
If you still don't understand.
Someone wants to buy it and....
You: Selling Spectral Tiger Mount. 10k gold, not a scam after the BoE fix!
Inc Whisper: Ok (or variation), I'll come and trade you.
You: Sorry, I haven't redeemed the mount on this server yet, because I am afraid no-one will buy it, then I'll just waste a code.
Whisper: Oh... ok, can you please redeem it now?
You: Could you please pay a deposit? I don't want to get scammed, it's worth over 1000 USD. :S
Use your imagination for the rest.
DON'T LURK DAMNIT