I'm unsure about League of Legend's accounts, though I'd imagine they follow the same process and protocol Runescape does in regards to account recoveries. So to reiterate on what's already been posted, always ensure the person you're buying from has a fairly credible reputation at least and (preferably) a long-standing history in account sales to minimize the risk of the account being recovered. Original owners can nearly always recover their account(s) at will and it's unfortunately become a very common and easy method of scamming users these days. Recoveries are mostly based on IP/ISP/Geolocation information stored when registering the account in question coinciding with credible, sensitive information relating to purchases made on the account. Always take extreme precaution when purchasing from any user and ensure you understand all the risks associated with account purchases before buying.