Anytime you are backlinking really it should go to your secondary or tertiary sites just to keep yourself protected as it was already mentioned. Secondly, I wouldn't buy in bulk, mainly if Google sees, or for that matter any Search Engine sees a single surge in backlinks you can be seen as a spammer. You should always drip links to your site, yes it takes a while, but worth it if you don't want to get slapped and put into timeout. The statement that "slow and steady" wins the race normally can be applied to SEO. Hope that helps.