I am in two leagues, one draft and one auction, both keepers, and am commissioner of the draft league. Although you said you are going to auction, I have found that preventing trades for picks until mid-June prevents some players from tanking or at least keeping decent players for a bit. Also, you can create a "Can't Cut List" to prevent it, especially if you are commissioner. Another way is to establish a date in mid-August that players must be on a roster by in order to be kept--perhaps the keeper date is a week after the trade deadline?
All in all, while you want teams to be competitive, you have to understand that if you are losing and intend on being competitive next year, you should do all you can to make sure you have as good a chance as any to get high draft picks and good prospects. You can also limit yearly acquisitions with penalties against the cap if they go over on the year. You can add caveats such as injured players and related movement not counting toward the cap, but if it is an obvious salary dump you can penalize them a buck a move, perhaps.