I use bookshop.org and have had a great experience. Not sure I'll try anything else.
also, bookshop is wonderful! we love them. from my other comment: https://news.ycombinator.com/item?id=44662335
> i think the modern indie bookstore tech stack should be bookshop.org doing it's thing + a website with your local inventory. let bookshop sell all the inventory you don't have on hand but can easily get from ingram, and then your unique inventory will rise above in SEO.