Either you are a Seller or a Vendor, to have greater readability, you should always stay under 200 characters. This will make it easier for the shopper to read through your bullet points and understand all the features and benefits.
You can find other rules on this page.
The first thing to do is to optimize your keywords.
Keywords are important to improve the visibility of your product, but you should never stuff your bullet points with keywords or keep repeating the same ones. You’ll want to make your listing stand out and to do so your bullet points must be easy to read, short, and to the point. The bullet points should address the benefits of the product first and every bullet should focus on a single and clear idea.
In your bullet points, you can mention changes made to your product, share regular updates with your consumers. You should write your bullet points for your ideal customer, personalize your description to reach the right customer base. But, do not stuff your bullet points with too much information and too many keywords, as you could lead the shopper to leave the page.
With long bullet points, you're not going to generate interest in the shopper but you're only going to cause confusion and, probably, boredom. Shoppers want an easy and seamless experience, so try to avoid information that is not necessary and go straight to the point.
First of all, you should always use relevant keywords.
Secondly, you should formulate bullet points from the customer’s point of view as you want to convince him the best way possible and you should never be corporate orientate. In fact, you should always have your customers and their needs in mind. Only by doing so, you're going to obtain their trust.