Mate before u trade an account, make sure u have all info provided so he cant steal you anything. 1st champs, friends, ip, original e-mail, everything.
Next time be more careful, if you still remember your previous accout details, send a message to support.riot. ofc you will not say that you traded or something, be clever on that topic, after that you will receive a message asking a lot of question, usually its 3 times, 3 different messages, then , if you answered correctly almost all, ull get your account back mate (usually a 80-100 hour process)
Hope you get your accoutn back mate, as for the dia one, im afraid its impossible if he hasnt gave you any info :/
By the way you can recall your account aswell (with Lol support) if you have the creation e-mail.