Only twice have I done something vaguely akin to this... I've posted an image of something I made (actually the bag in my avatar) onto imgur, and posted it on 2 relevant subreddits with a title like "fuck yeah! just crocheted and embroidered this f7u12 bag!" I thought it was fine because the focus was on my excitement over doing this; not on selling the item. Very important distinction! And I did this by posting one image; not linking to the etsy listing.
The good news is a few people in the comments asked if I sell stuff like this. THEN I was fine posting my shop URL in response to them. I also got some PMs about custom orders, two of which ended up working out.
I tried this again with my trollface laptop sleeve, but it never got beyond my one upvote. I assume it's because I posted it at a wrong time or something -- because it's awesome! :)
So my thoughts: reddit is NOT for shamless self-promotion of your shop, but it CAN be a forum to show off a picture of your favorite craft (in fact, that's what a lot of subreddits specialize in, such as hookandyarn). Then if people inquire in the comments, by all means share that etsy url! But don't be pressuring redditors to click it.
And those are my two cents. :)