If the site hasn't already aged for at least a few months, let it age until it's at least a few months old before you do any serious link buying. In the meantime, create as much high-quality informational content for the site as possible. Make sure you link internally where relevant between those informational articles and some of your key product pages. Then start buying links. A combination of niche edits and guest posts works great. Send some links to your homepage with branded anchors and send even more links to your informational articles using a mix of exact match, phrase match, generic, and branded anchors. Your internal linking will take care of the rest. You can send a few links here and there to your product pages, but be careful as you don't want to trigger any alarms with Google. Make sure it's natural.