Buy/Resell at leisure. Once applied, the discounts last until used. So, very useful if creating your own on-track and haven't researched the tanks, yet. However, the discounts do NOT stack with any other sale/discount that WG may offer.
For instance, this year, I've used/bought/resold all my choices for Tier II-VI, used my discounts on Tier VII/VIII PL line, and used/bought the Type 4 and T-100. I'm still teching up the PL line (on tier VI), and while I could have made more credits on the VII with a different tank, since I need to grind the fucker anyhow, it was wiser for me. As the VIII