I've been contacting Jagex via email, and they basically told me: we know you are not the original owner of the account, the account must of come into your possession somehow, account sharing isn't allowed, its staying locked.
it would appear so, I've never seen this occur in my on and off 7 years of RS, I've played and used bought/gifted accounts tones over the years and never had any problems.
then i login into one of my level 3s i bought 1.5 years ago, make it a member, boom locked because I'm not the owner.
My guess is the person you bought it from had his IP flagged so any accounts that were made from his address which aren't on his adress anymore are getting locked.
Have you ever moved houses or changed your IP for any particular reason?
Jagex tends to lock the account if a foreign IP other than the one created on ever logs into it.