Or maybe limit the number of sales transfers.
A team usually only makes money from one home game per week, and is limited in the number of players it can train, but there is currently no limit on how many transfers can be made.
A transfer fee,may reduce profits, but it doesn't necessarily stop DT. If I only make $10,000 on 20 transfers/week instead of $20,000, my profits are simply less.
A holding period might require an owner to spread his activities out, plus take on market risk and salary costs, but it would still be possible to possible to sell about 1 player per day (98 per season).
A limt of, perhaps, 3 sales per 7 days, and 10 per 5 weeks would basically allow a team to go through 2 complete roster cycles per season. It would let someone who bid on 3 players and went to sleep to get rid of the extra one or two, And it would let someone engage in modest trading, such as 9 players over 3 weeks.