I've seen a lot of rollbacks in the past 2 months. I would think with only buying 300m, you should be on the safer side of things. But still trade with caution.
Unfortunately, with the release of botany bay, they've increased the level of rollbacks by a huge amount. They're mainly just wiping or banning the accounts, so watch out!
Be smart about it, don't initially trade with one of your prized accounts and wait a few days to a week before transferring the gold just to be extra safe. Doubt they'll even peep at 300M.