When I am buying BTC f-to-f, by cash how can I know all the outputs of the senders's wallet are from legal sources?

Hello,

If I purchase BTC by cash from unidentified person (found at localbitcoins.com) and deposit it directly on my cex.io address, am I violating terms of use?

I already read cex.io/terms but it's not clear if there is any issue with this specific activity.