If you're an enchanter, you prolly know this already... but every so often I see people in trade asking for an enchanter to link in trade... NOT for an enchantment or whatever, but just link... so, I take the 2 seconds and link it...
15 minutes later, when I'm in a dungeon, or halfway into a queue or something... I get whispered a random list of enchants that add up to about 50g of mats from this complete stranger with the promise that "they'll even provide the mats", or the even better one "Do I have mats on me?"
At which point I reply that I inform them that I don't craft for Trade. Even *if* they happen to be the exception to the rule and tip me afterwards, a 20% tip off a 100g enchantment doesnt even equal the same amount I could get from running and killing Sha for a bag of gold in the same amount of time it'd take for me to wait for this guy to buy the mats off AH, trade me, and then get the enchant. *if* everything goes smoothly, which it usually doesnt and I end up having to explain why I can't make enchantments from these 2 stacks of linen cloth they just traded me.
Think about the person's time. If you can't afford to give the person 100g per 5-10 minutes time spent dealing with you... just go to the AH or mail a guildmate with the request. (I'm very proactive about making sure my guildmembers get what they need for free, but wasting my time is a fast way to get on my bad side, too)