Think about it, if you were law enforcement what would be the point of setting up a market? You can't identify vendors, you can't identify customers using pgp. However, if you ran a tumbling service you could potentially identify a lot of users. Now obviously there would be no way for LE to prove that you are doing anything illegal, but if you regularly move money through a tumbler you might start getting some extra attention.
Vendors I hope you are smart enough to use some other method to obfuscate your earnings.
As to the small buyers out there you'll probably be fine. But if you want some ideas, one way would be to use an exchange such as btc-e. What you do is create an anonymous account on btc-e move your coins there, make sure it's in their main wallet then move it through a tumbler. That way both organizations would have to be compromised in order for LE to track you.
Everything that people use should be scrutinized.
The biggest problem inherent to using markets is the fact that nobody can possibly be an expert in everything that is needed to use markets safely. Most people are able to be sincerely aware of what they're doing and understand what areas they don't have quite as much expertise in as they might.
For the most part, DNM users only have enough of an understanding about the entire BTC economy in order to use it to use DNMs, and it gives people a terribly slanted view of how much of a "red flag" they might cause, and put a lot of time into worrying about how to minimize the attachment to their name. The fact of the matter is jumping through a ton of hoops in order to disconnect one's name from BTC transactions is in itself what looks suspicious, since DNMs are only a very, very small fraction of all BTC transactions. Money laundering is the far greater concern, not buying drugs.
I make a not-insignificant amount of income buying and selling bitcoins. I keep appropriate records, and have no concerns about if I were to be audited, that it will somehow betray that half-dozen transfers that I make over a year to DNMs. I have a better understanding of how wallet addresses themselves are more than adequate to make up for the public nature of the blockchain, and if you read criminal complaints or related articles, even the largest of the cases involving DNMs all show that examination of the blockchain adds absolutely nothing to the investigations.
I am not saying that tumbling isn't a precaution worth taking. What I am saying is that people focus too much on the wrong things at some times and it opens them to other risks by not considering how some of the associated actions they're taking may be placing them at further risk.
I sell in person on LBC. I, and pretty much everyone else who does, understands that a decent portion of my buyers are going to send the coins off to DNMS. We don't give a crap. However, most of us will refuse buyers from Tor or who sound overly paranoid. We are held to TOS that says that we agree to not sell to buyers who we suspect may be engaging in illegal activities, and we have to stick to that. But we counteract that by being completely disinterested in why people want BTC. It's a don't ask and don't tell silent agreement that almost everyone understands. So acting paranoid or accessing via tor, or asking questions like "you can't see where I am, right?" is exactly the kind of thing that raises eyebrows. We sell bitcoins, so someone buying bitcoins is the exact opposite of unusual.
Where I am going with this is really that people ought to take the time that it requires to get a better understanding about the financials before wasting a lot of time or risking themselves by jumping through unnecessary hoops. Ultimately this isn't where people get in trouble, and by feeling falesely secure by completely obliterating any connection from a name to a BTC transaction, it allows people to take risks that they shouldn't. People somehow forget that it doesn't matter if LE is able to prove they bought drugs on DNM or where it came from --- possession is having it, not buying it.
I do appreciate BTC isn't a simple concept. I have been involved in them for about four years and there are still a lot of things I am unclear on. And I am not at all saying everyone needs that level of understanding. But I think that gaining that knowledge makes it easier for people to understand exactly what you are warning against, as I feel it is probably as not clear to everyone as it should be (It is very well-written, and I want to compliment you on that. )