I was thinking about it. If you make a Lbc account using clearnet using your identity and then go onto withdraw the bitcoins you purchase using Tor I think there's a chance you could come under investigation. Lbc could highlight to LE that someone has withdrawn bitcoins using tor and therefore this is suspicious. They would give your clearnet ip address to LE and they could 'keep an eye' on you. Whilst you have done nothing wrong after that (tumble, tails, pgp) the fact you have used tor to withdraw could have LE knock on your door for a chat. What are the chances?
Hang out at /r/bitcoin for a bit.
In short there are two points you should keep in mind. Three actually, the obvious being buying and selling bitcoin is perfectly legal.
There is simply no way LE is going to start by looking at a transaction via tor and build a case backward from there. The amount of resources it would take is astonishing.
LBC, a company that capitalizes on promoting the trade of bitcoin is fully aware of their many uses. They're unlikely to shoot themselves in the foot and lose a massive amount of business by doing that. The success of bitcoin owes much to the privacy and anonymity of the transactions. Telling their customer base: "trying to protect your privacy further will result in us reporting you to the authorities for doing absolutely nothing wrong," will cost them a LOT more business than people hoping to use DNM. The bitcoin community at large would riot.
Sellers have the option to not make trades from people communicating via TOR, as high volume traders worry their transactions may be scrutinized. LBC doesn't care how you access their service.