I've never sold on eBay (but have bought one or two things

), so no firsthand experience here.
Looks like there are a couple of threads that mention onesource if you do a search on
this forum, as well as other info that might benefit your son. This is a UK-based forum, but no doubt much of the discussion applies globally.
(You may already know what follows; if so, just ignore). Chris Malta is apparently the "brains" behind onesource as well as its sister operation, worldwidebrands dot com. You might want to include this info in your research.
Here's a lengthy thread on yet another forum, discussing his directory (haven't read it, but sheer length suggests it might contain usable info).
Not that authoring a book automatically confers credibility, but the
Amazon reviews from buyers of his book may help give you a sense of what he and his companies are about (see "35 customer reviews" just beneath book title at top of page).
Whoops! Noticed your profile says writer/editor. Ignore the credibility comment.
Hope this helps.
Dan