Yep, aggregating on your own (manual/scraping automatically) would be the first steps to gain initial momentum.

Then once it looks like its not a ghost town, let people post for free in the beginning (they would again only post if they think they stand to benefit from posting).

Then try getting it featured / covered in blogs / press.

Then once you see traffic starts building up or people start posting jobs on their own, introduce paid listings for highlighting certain posts to stand out.

And once you have managed to keep it going, turn paid listing with a small pricing which you will keep on increasing regulating the supply-demand.