It obviously depends on the merchants and the product categories. There is no easy answer to this question. Some merchants have millions of dollars in sales per year, others hundreds of thousands, and some a few hundred dollars per month.
You can find or deduce the number of sales of a seller by looking at their number of reviews, or their profile on marketplaces. This information is sometimes published.
You can also deduce the sales potential of an article via its “Sales rank”, information published by Amazon on each article file.