I don't know whether it's something BSD-specific or not, but rs can do this pretty elegantly too.

$ whatis rs
rs (1) - reshape a data array

Invoked as "rs -C, 1" it will convert an input separated by newlines to a comma separated output:

$ grep '^oxym' /usr/share/dict/words | rs -C, 1
oxymandelic,oxymel,oxymethylene,oxymoron,oxymuriate,oxymuriatic,

Plus you can undo the transformation too, if you pipe the comma separated input through "rs -c, 0 1" (lower case c).

The downside is that rs will leave you with a trailing comma, but as long as that's not an issue it's a pretty neat trick.