First of all, welcome to the forum, Eric!
Frivolous talk is speech that does not contribute any value, either in the mundane sense or in the lokottara sense (helpful towards Nibbana).
The example you gave about weight loss is not frivolous talk; that information could be beneficial to someone who is trying to lose weight. So, that is beneficial in the mundane sense. When we meet a friend after some time, we ask about one’s health and one’s family and that is not frivolous talk.
You asked: “ If I “make conversation” with a customer about sports and the weather, is this frivolous talk?”
No. That is part of your responsibility (and courtesy) as a salesman in order to win the customer’s confidence and sell merchandise. Of course, you would not want it to be dragged into a lengthy conversation about weather or sports; that would be frivolous. So, you need to make a judgement about that.
Frivolous talk is when people get together and talk gossip or crack jokes about others for hours. In general, talking at length about things that are not conducive to the benefit of anyone is frivolous talk.
In general, the following can be used as a guide:
– If you know something that is not helpful and is untrue, then do not say it
– If you know something that might be helpful, but is untrue, do not say it
– If you know something that is not helpful and is true, do not speak about it
– If you know something that is helpful and is true, then find the right time to say it
Of course, discussing Dhamma is the best type of talk, since that will help everyone immensely not only to elevate “the quality of life” in this life, but also to work towards getting rid of samsaric suffering and attaining permanent happiness (Nibbana).