I looked at libro.fm now, but I don't understand what I get if I buy an "Audiobook" over there. The word "mp3" seems to appear nowhere on the site.
The situation is similar, though slightly better on downpour.com. At least they say "digital download". But not what you can download after you buy. They mention mp3s on their FAQ page. So it might be worth a try. But I would feel more comfortable if I really knew what I buy before I buy.
Downpour lets you choose between an .m4b (an Apple format using aac audio, basically an mp4) and .mp3's.
Both websites say explicitly in the FAQ that all downloads are DRM-Free, although libro.fm makes a bigger deal out of it. If there's a screenshot or something that would make you feel more comfortable about what you're getting, let me know...
Is there a good place to buy audiobooks as simple MP3s?