I mainly use the official Splinterland marketplace but secondary markets such as PeakMonsters are awesome. They have allot of great features not offered on the main Splinterlands site. I believe some (if not all) the secondary markets do offer "discounts" when using their site to make purchases (check their official sites for complete details).
P.S. And if I'm not mistaken your referrals' ongoing purchases also earn you future credits in @Splinterlands (except if they buy packs with DEC/credits).
Yes, affiliate earnings are ongoing. Not all purchases count. To the best of my knowledge Pack/Potion/Spellbook purchases all count towards affiliate earnings. Buying a single card from market does not.