In a short term sense, yes but in the longterm the culling of sellers drives up the profit for the remaining sellers.
It's in a sense self defeating.
You ban (some) suppliers
Demand goes up
People join in because it's that lucrative
That's why im arguing that buyers are the root of the problem, they create demand, suppliers merely statisfy the demand.
Seems pretty obvious to simply check the inflow of gold on an account rather than a hard amount in a single transaction.
Asking about specific numbers is also a bit silly because Blizzard likely already has set some numbers internally that sets specific flags, it's not a terrible leap to extrapolate timeframes from then on.
The obvious thing is that you've obviously interacted via chat with the GDKP host at some level and joined their group.
Tracking the amount of bosskills that you've done with a given account is naturally possible, which makes it more of a hassle to replicate for goldsellers.
Let's be honest, if your system is fooled by a note that reads "thanks for helping me out
" then that system is not terribly good.
May be slightly offtopic (but still kinda related): perhaps putting up items with the deliberate intent of fooling someone into buying them for huge amounts should either not be possible or also result in a penalty when repeated offense is happening.
Barring that you can also reverse engineer the whole thing, say they've managed to track a sellers account, what's stopping them from looking at all the accounts said account has mailed gold to?
If you received substantial gold from an account that was used to sell gold, then those accounts likely require an investigation.