There's not a way to stop inflation of player selling prices when selling directly to eachother.
Even if there was an in game system where say you put a 3 mil small plot for sail, for a cap of 5 mil let's say. Somebody could still easily say 'I'll sell you this plot for 20 mil plus the in game 5 mil cost.' The only way to stop that is to make it unreliable that any one given person will get the plot after it's vacated, which is what the timer is for. The only issue there is you can STILL sell directly to people who already own plots, as they can immediately transfer in when you vacate it. Though that does at least leave an open plot in its wake for a person or fc with no house to buy later.