The most I have bought at once is 250m and that was very recently on OSRS without issue, traded it via two mule accounts and had no issues. Jagex should be able to easiliy detect these things but they tend to go after the sellers rather than buyers, don't use a VPN as this might actually increase your ban as if you trading on different IPs they might actually think one of your accounts is the seller, you can trade junk put to be honest I reallt doubt it makes a big difference given values have been tracked atleast since the introduction of the GE.
I have transfered 100m many many times, I've bought about 500m in the last few months without issue, I'm sure Jagex know I bought it...bonds are too expensive.
Once more thing, don't use a fresh level 3 account to mule the gold, on OSRS (maybe rs3 too) the trade restrictions on new accounts mean you will be able to recieve the gold but not trade it off the mule account until you either hit certain skill/quest requirements or become a member, very wasteful to buy a bond on a fresh account which you have 0 use for. Just find an old account to mule, as long as it was created before a certain date restrictions won't apply, you can easily test it to see if it's trade restricted.