I've had a good four years publishing via Amazon KDP but in the past six months my sales have fallen off a cliff and nothing I do reverses the trend. Nor can I get any sense or helpful advice from the support team, so it's time to move on. I've noticed other authors talk of 'going wide' i.e. publishing via Kobo and Barnes & Noble as well as audiobooks. I've started transferring onto audiobook via ACX but would appreciate some advice re other sales platforms. Are they as user-friendly as the Amazon format? Can I reasonably expect decent sales? Thanks.
Many people here use D2D (Draft2Digital) to reach other platforms. I've never had any problems with them and their customer support is good. They distribute to B&N Nook, Kobo and iBooks, as well as some other platforms. As noted, Amazon is still the largest retailer, but assuming you're not in Select, there's no reason not to try the others.
It's easy to get onto B&N/Nook and Kobo. It's not exactly the same process as KDP but close. On the other hand, iTunes is distinctly user-hostile. I got onto it by myself (an epic quest, and I'm a Mac user), but for most people an aggregator (like Draft2Digital) makes it far easier.
I think my non-KDP ebook sales, ACX audio sales, and paperback sales between them are roughly half my book income most months.
Have you tried a Bookbub promotion? It's the only thing I've found that really gooses your sales, though it is expensive and hard to get accepted. (Other lower-cost and lower-yield alternatives include ENT.)
Are they as user-friendly as the Amazon format? Can I reasonably expect decent sales? Thanks.
If you know how to build a good ePub file, you can use that same file everywhere. So yes, getting onto the other retailers is every bit as easy as publishing with KDP.
As others have said, I recommend D2D as long as you're not publishing erotica (or maybe even erotic romance). If you are, then use Smash*words.
However, my experience has been that books not selling well on Ama*zon will sell worse elsewhere. There's no magic bullet. The best thing you can do to boost sales now would be to write and publish more books.
Overall, publishing elsewhere is no more difficult than through KDP. Just different hoops. But the physical act of publishing isn't the hard part, is it? It's marketing. Most people find it very difficult. Even if you don't, now you have to split your marketing efforts across multiple platforms. You'll have to market just as hard as you do on Amazon, but competing for a much smaller base of readers. Also, it will take time to gain traction there, just as it did on Amazon. All things to consider. Good luck no matter what you choose.