I think what you describe is fine, provided you don't actually give BitInstant your real identification and access it via Tor somehow. Otherwise I'd think it can be tied to you if they try really, really hard -- but I've never actually tried to do that, so I could be wrong here...I think it comes down to "how paranoid are you," in the end. It sounds fine to me, but if you're a bigshot vendor -- well actually, if you're a bigshot vendor, you wouldn't need to buy coins, would you...