What is conning people?

I think I understand it under some terms, but I would like to know the exact definition because I am very good at buying and reselling and I am only 15. I have made several hundred basically buying (sometimes using) and reselling. Am I considered a con? I dont really lie but I talk all on positives and maybe one or two negatives. For example I bought a PS3 for 220 and sold it for 330 after buying a hdmi cord for 15. I also then bought a used tv and sold it for nearly double what I paid with nothing different. I am not sure how I am a good salesman maybe? or a con artist? IDK? I want to be a cop one day and not get in trouble..... so morally and socially I am wondering if I am considered a con?