For Etsy be sure you do not have a book long description for your items. First of all no one reads those anyway and secondly they look terrible on a mobile device. Make sure that your best keywords are in that first sentence of your listing description. This is what most mobile device users are going to actually read (and what gets you found in search) Anything after that is fluff.
Now I list a lot of yarn so I do have to put the details as this is important to knitters and crocheters. But most items just need to say what they are with your top keywords.
Adding a link to another item in your Etsy shop is good but don't get carried away. I have seen some that list 10 or more links to other items in their Etsy shop. That is overkill and it will hurt you in Google and makes it hard to read on a mobile device. Remember many times less is more especially for mobile device users.
For both Ebay and Etsy listings be sure your pictures are tight. That means no huge white/empty space around your pictures. Take a good up close picture of your item and make sure it adheres to the Etsy or Ebay picture requirements. On a cell phone pictures that are surrounded by a lot of empty space are very hard to see. Or pictures surrounded by a busy background. I know we all hate boring white but it really is the best background.
In my opinion Buy It Now is much better for people purchasing from a mobile device as they don't have to jump all over the place from screen to screen.
Get rid of the fancy backgrounds. Ebay is also telling people to stop using them. This will really hurt your listings on Ebay mobile.
If you have an Ebay Store do put one link it to your storefront or to a companion item. I am also working on getting these in. When I first got my Ebay store I didn't know you could use the insert tool to put in links. This makes it easier for all customers (but especially mobile customers) to look at different items you sell.
Do use item specifics! Even if you are selling handmade be sure to fill in as much of them as possible. This is getting more and more important to search on Ebay
Use a 12 pt font or bigger (but not huge) I use 14 pt and I center all of my descriptions and put them in bold black. This looks really good on a cell phone.
For both Ebay and Etsy DO NOT put your policies in your descriptions! Just stop doing that! It really hurts your listings in search and this will become more and more evident as time goes by. Both Etsy and Ebay have specific areas to put your policies (shipping, guarantee, etc) Use only those areas. Your listing description should be just that - a product description!
Wishing you all great sales this holiday season!!