これ良く売れてるんですか？って英語でなんて言うの？

Is the product selling well?

This is simple way to ask if a product is selling well.
For a more open question you would use, “how is the new product doing in sales?” because you start the question with ‘how’, the person will be required to answer in more detail instead of a simple yes or no.
If the partner has sale figures, you can also ask like this, “Do you have this month’s/week’s sales figures for the product?”
Then you can clearly see for yourself how the product is selling.

Since this is in business terms,
Demand = Need,so the higher the demand the better the sales for that particular product as the people are buying it.

Since this is a business partner,it is important that you use the correct vocabulary to express yourself. You can use the following sentences:

1.How is the demand for the product?

2.Is the product selling well in the marketplace?

"Hello again Sam. I wanted to ask about the new product. Is it going well?"
"Yes, I have some early figures and all the signs are that it's performing in the marketplace very well."
"Excellent, I'm so pleased to hear that!"
"Yes, we are well satisfied with the results so far and the projected sales figures are considerably better than expected."

"Is there a demand for this product?
"Do we have clientele who is interested in this product?"
"What are the sales like of this product?"
"What is the turnover of this product?"

These are simple ways to check with your business partner who is more hands-on in the shop if a certain product is selling and he will be able to tell you how many customers purchased it and what the client base and sales are like for this product and if there is a need to order more stock or a high demand.

Good demand' is a business English phrase used to describe the positive level of interest customers show in a product or service. You could also say 'high demand' or simply 'demand'.