Hi Apocalypse,
Leaving aside whether that concept should be a rule, a well written rule has crisp boundaries that are easily identified by all readers, even readers for whom English is a second language or whose grasp of English is poor.
"If it makes you money afk, don't do it" would be a poorly written rule. It's too vague. What makes money? Selling to vendors? Crafting items that can be sold to vendors? Enhancing abilities which permit you to succeed at quests which earn gold? Posting messages to world chat advertising your money-making vendors? Where is the outer boundary of the group of activities which "make you money?" Far too vague.
Write concise rules, not vague ones. Rules that don't demand further explanation. If you don't want folks selling to vendors or on exex, say "no use of a macro to directly sell anything while AFK." Don't say "no selling AFK" because your vendors and exex can sell for you while you're AFK or even offline. Crisp borders, not fuzzy ones. If you don't want folks making stuff AFK they can later sell, say "no crafting AFK." And if a few activities remain which you want to disallow but they don't fit in to obvious categories, state them explicitly.
Don't make players guess what the rules really mean!
-Wil
One of the problems with society today is with people who demand everything spelled out, so they they can then try to circumvent said rule by a technicality. We want some vagueness there so we have wiggle room where we feel it is necessary. For the vast majority of players a rule like 'if you can make money doing it afk, don't' is plenty sufficient. It's generally understood that using an in game feature such as a vendor isn't covered by this rule, unless someone is being extraordinarily nitpicky. If players have questions, they can always send a page and ask.
We can't cover every possible situation so vagueness is a viable solution. If you don't think you should be doing something, you probably shouldn't. If you think about the meaning behind the rule, and think well maybe this is what they mean, by all means send a page and ask if it's ok before assuming it is.
In your example, selling to vendors, crafting items, and chat advertising would probably all fall under that rule. Skilling however would not, as we provide a training room up to 75 skill, and personal trainers up to 120 for *most* things. It would be vague on skilling alchemy or something, but you could always ask our stance on it. Personally, keg making/filling should be against the rules. I've brought it up many times, and hopefully with this post, it will finally see resolution.
Consise rules are easier to get around or otherwise exploit. If the rule was "You can't stand inside your house to kill monsters while afk", someone would have a script to move them around on the porch, or move around the house, or try to cast energy vortexes from the roof, or... whatever... "No killing monsters while afk", while a lot more vague covers ALL of these scenario's and more.
If we tried to cover every possible scenario, the codex would be 700+ pages long, and just overwhelming, and not very useful at all.
Again, if you have a specific question on something you want to do, and aren't sure if it is breaking a rule, SEND A HELP REQUEST AND ASK BEFORE DOING IT.
Worst case scenario, I have to ask +Nyx or +Veritas, who may have to in turn ask +Colibri. So a bit of a waiting game, which doesn't really harm anything.
I find this confusing. 
The codex has the following to say about AFK activities:
Like if AFK macro-crafting was against the rules then I'd expect spinning, weaving and tailoring to be covered by that. But macro-crafting is not listed. Nor is any other activity whose plain English understanding would reasonably including weaving yarn into cloth.
