if u only had a scale on u and no baggies you would have a defense against the distribution charge: I use the scale to make sure i get what i pay for, period. that is ofcourse if the amount of weed u have on u can be put into criteria of personal use, i would say most places 1 or 2 oz is the limit.

if u got 18 grams of marijuana, a scale and baggies in your car, you are seen as a drug dealer in the eyes of law and u have almost no legal defense in such a case, although u can use the story u mentioned above, i doubt any lawyer, judge, jury or officer will buy it.

plus depending on state, there might be a Delivery charge as well, even if u r truthful and give it away and dont sell.

hell even just a scale alone, along with some baggies and some cash and WITHOUT ANY ILLEGAL MATERIAL (substance or paraphernalia) constitutes to "contraband" and is going to get u some sort of charge. although that is easy to defend with a good lawyer. I have seen people who have been charged just for this very same contraband only thing.

I'm no legal expert, u should always consult with a lawyer for a firm and correct answer to such questions.

good luck