The Guild Bank method
Hello!
I thought I'd offer up my thoughts on this. ^_^
First off, as above, have your guild master in a completely separate account from any of your gold botting/trading accounts.
Second, set up your guild ranks like this...You can name the ranks however you like, but set up the privileges like this...
Guild Master
Second GM (has maximum privileges in all tabs, gold withdrawal, etc)
Third GM (same as above)
4th, etc...however many levels you want. I use 3 and have found that I sometimes need more.
After your GM levels, create a trading rank that has no permissions except to withdraw max gold per day (100,000)
and finally, have an initiate level that has zero permissions at all.
NEVER HAND GOLD OVER OR MAIL IT!!! Your buyer needs to join your guild, you'll upgrade their rank, and they'll withdraw however much gold they are buying.
*NOTE* If you don't trust the buyer, make sure to withdraw all but the amount of gold they are buying from the guild bank.
*NOTE 2* This WILL cause problems if you're trading more than 100k gold, but there are workarounds to that! ^_^
This method creates the cleanest disconnect between your main/guildmaster account and your botting/trading accounts and, so far, seems pretty safe.
As a bonus tip, for those that are unaware, Blizz has, at some point, added the ability to take over an idle guild. When my last batch of accounts got banned ~8 months ago, I took a break. When I came back, my accounts that weren't banned were able to "dethrone" the old guildmasters and take over the guild.
Unfortunately, if you don't have any ties to the gold on the banned accounts, I don't think there's any way to retrieve it. (To answer your original question)