Are there any legal implications when using localbitcoins.com?
I'm not a lawyer. In the U.S., the lack of mens rea in your example would not cause you to have violated any laws.  As a person selling your coins, it is not your responsibility to determine your buyer's intentions.  

Of course, this is not quite so simple.  Money transmitters have regulations that include AML/KYC (know your customer) requirements which might be a factor here.   FinCEN has asserted that the conversion of bitcoins to and from cash (or other substitutes for value) is transmission of money.  So if you are engaged in trading as a business (i.e., not as an "infrequent" trader) you would likely be expected to be registered as a money service business (MSB), be licensed as a money transmitter and follow the AML/KYC requirements when conducting those trades.   Those requirements would involve gathering identity in certain instances, keeping records (for five years), and reporting suspicious activity.

Now, whether law enforcement will pursue a trader for trading without registering or following AML/KYC is unknown.  There are multiple individual traders whose banks have taken action to shut down service to their customers whose banking activity might be linked to providing Bitcoin exchange services.

So regardless of whether there is legal risk, there are other hurdles when engaging in trading to consider.