Well for keeping track of profits, you could open up a word pad document and write your prices each time you buy something.

And also, you don't want to lowball to hard. Because it will frustrate people and make them not want to do business with you.

And know what prices are, learn what things sell for. Also, buy things that are popular like you said, Hunter joints. Things like supernova and Beetle aren't very popular so you want to avoid them.