I think there are so many ways the game can stop rentals that the problem could be easily fixed if they decided it was a problem.
1) Game shape penalty. After a player is transferred they lose two levels of game shape reflecing the stress of moving and the getting used to the new team, their players and their plays.
2) Eligibility. Make it that players can't play in playoff games, relegation games or top 16 tournament games unless they have been on the roster for at least two weeks previously.
3) Hard Salary cap. I don't agree with this but there could just be a hard limit. If you try to bid on another player (would also count any players you have a current bid on to stop tricks) it would say "this will take your team above the salary cap" and not allow it. For JBBL somewhere around $800k for division II around $500k.
4) Soft salary cap. You can go above it but at financial update time for every $ you are above the cap you have to pay $2 in luxury tax. This is then shared amongst every team in your league that isn't above the cap. So if the JBBL cap was $800k and you had a $1 million roster you would be paying $400,000 in luxury tax which your rivals would love to share. Can still be done but you would need a good reason.
About tanking, it is so easy. The salar floor is $227k, no real team would be bothered if it was $300k but the tankers would at least have to have a reasonable roster.
One funny thing, I am so close to average.
Player Salaries: $ 535 144
League Averages: $ 536 228
Staff Salaries: $ 88 029
League Averages: $ 87 334