1. Jagex can see you are using a proxy
2. Acc creation/the start of the account is probably the part that Jagex spends the most effort on in terms of detecting bots/suspicious behaviour
3. I highly doubt the ip was never used on osrs before, it shouldnt make much of a difference because 1. and 2. should be the bigger factors, but still seems kinda scummy by the seller
Ips dont really get flagged and if you only bot one account at a time you can just bot on your home ip