In terms of audience, I believe BrandBucket has more potential and it also has appears on top Google ranking for the term 'brandable domain name.' Then again, BrandRoot is also a known site for selling brandable domains. Namerific, I don't think so because the site has no solid track record. So yes, it's kind of tricky how to decide which site to submit domains to.
But what I have read here about BrandBucket is that the site has gotten too crowded with domains and it could take a good chunk of time to see any sales (unless you are lucky, lol. Just kidding.)
So, my opinion would be to go with BrandBucket after all, as at least you can expect to get some exposure for your domain(s).
Good luck,