A lot of people still have questions on this topic (http://pages.ebay.com/sellerinformation/news/sprupd16/view-item.html) and a handful of you asked me to create a dedicated thread, so here goes...
For a while now eBay has been optimizing item descriptions for small mobile screens by showing small portions of the full descriptions based on keywords - the full description is still viewable but the link can be hard to find.
With the new changes (which I think are good) eBay is giving options for sellers to optimize their own descriptions for mobile listings and they are also making the link to view full description much more prominent.
Here's how it works:
eBay, based on keywords, will automatically select a limit of 250 characters (not words) from your description to show on mobile view. This is something they have been doing for a while now. In the example they provided via the Seller Update, that description was just 139 characters. They could have included the second sentence in the provided example and still only hit 230 characters, but the bots only picked up keywords in the first sentence so only 139 characters were displayed. So again, they will show UP TO 250 characters.
Mobile view is also being enhanced by including a prominent "Read more" link at the bottom so buyers can easily click through to view the full description. here is a screenshot as provided by an eBay Blue:
If your full item description is less than 800 characters (I am reading that as 799 or less) and uses HTML, CSS, or text-only then eBay will, by default, show the entire description of up to 799 characters instead of pulling out text to fit within the 250 limit.
Some people have asked if eBay can pull 800 characters for mobile why are they defaulting to a limit of 250 if your description is more than 800? Faster load time would be my guess.
You don't need to do anything and eBay will optimize your descriptions as noted above by default. Again, for a while now eBay has already been pulling summaries of up to 250 for mobile, but now the link to view full description is going to be more clear and if your description is less than 800 characters they will show the full description by default. More info about how to optimize your listings without customizing text here: http://pages.ebay.com/sellerinformation/news/sprupd16/view-item.html#tab=what-you-need-to-do
eBay is adding the option for you to choose your own text for optimized mobile views. If you DO want to customize then keep reading.
If your listing is 800 or more characters you have the option of using html tagging to choose which part of the description is optimized for mobile but the text needs to be consecutive - like you can't choose a sentence from the top of the description and another from the bottom. If you choose to do this, the html that can be used is shown on the FAQ page. http://pages.ebay.com/sellerinformation/news/sprupd16/view-item.html#tab=faqs&faq=faq-vi-q09
General html tags (like tagging for a bold font) do not apply towards the character count. HTML tags that use up space (like a space between characters or a line break) do count in the 800 character count. A line break counts as 50 characters since that's about how many characters will display across a line on the mobile view. If you are using something like a Word document to count characters for you (no word yet on whether eBay will provide a character counter), please keep in mind how many characters a line break takes up.
If you tag more than 800 characters, eBay "will truncate the selection to below 800 characters. We will remove whole words to get below 800 characters, not partial words."
The official roll-out is this summer (I believe May 1st), but if you start html tagging now there will be "no visible impact on your listings". So if you want to start tagging now you can, it just won't go into effect yet. Do keep in mind that eBay has said they are going to provide some tools to help with this. They have not said what those tools will be yet. I made a suggestion that eBay implement these types of tools (and I hope they will take it under consideration): http://community.ebay.com/t5/2016-Spring-Seller-Update/Suggestion-for-Mobile-Optimized-Descriptions/...
I honestly do think this is a good change. It's enhancing the way mobile is currently handled and it's also giving sellers the option of customization if they so choose. Now, if only they would fix that "free shipping" glitch... 🙂
Oh thanks Copper - I tried to look her up recently but until I saw this old thread I forgot about the asterisk in blackbird. Will send her an ebay message! 
