I'm not sure if this is already widely known or just obvious. This is just a strategy to get BNDS for basically free. (This depends on who you already have on your team and if they're auctionable.
I paid 1.6 million for 2 98's and put them into Deion 99 Set. I got back around 1.1 million from the auction after selling the 99. I kept the BNDS and auctioned off the players who got replaced. The reason this is a good strategy right now is that a 98 is basically an endgame card with chemistry boosts so they don't really need to be replaced. By selling off the ones who got replaced, I got back like 600k and then had 1.7 million. I repeated this strategy with ONLY Deion and Moss because they're the most expensive 99s and therefore sell more. I ended up with 1.3 mil (I had this before the I did the sets) and 8 98's that are BND. If you try it with Gonzalez it's more expensive and you end up with less because he sells for less.