I've struggled with this as well. I do a lot of "cruising" and by looking constantly at others' photographs I have a good idea of what I love. Though replicating it is another story :)
First off I really do like how some shops mix up their presentation with the first photo, so I try to do the same. It makes the shop interesting and appealing to me, and I want to stay :) For instance many are of just my work, some are with the piece laying flat against a white board with a paintbrush, and others are of it framed. My favorites I've seen are with the artwork laid out with various still life items around it. A great example I can think off of the top of my head is River Luna's shop, like in this listing:
https://www.etsy.com/listing/91042489/watercolor-feather-painting-archival?ref=shop_home_active_14 and another example here:
https://www.etsy.com/listing/205924947/tiny-tuscan-landscape-original-collage?ref=shop_home_feat_3I LOVE these though understand it may be personal taste! I've tried to do something of the sort with my paintbrush and tubes of paint. Can't seem to get them quite as beautiful, though! The rest of my photos when you click though usually include the entire artwork, maybe one of it framed, and one in which I do my very best to show up close with possible texture.
I think the up close and at an angle to show what texture you can is very important as it gives that "in your hand" feel. You could imagine holding it and touching it. I'm still tying to accomplish doing this with a camera successfully because as a customer, shops which display only the scanned or printed image and a frame just don't do enough for me. I want to know what it would really look like to my eyes... if that makes sense.
As far as watermarks go I'm pretty neutral. I don't use them but they also don't put me off either as I understand why some do.