Using the player with 2k-10k ET range that goes for 30k a half hour after the bidding was supposed to end as an example:
Whatever a player sells for is his fair market price. This excludes what is known as "overbidding", where a team clearly bids more than he is worth (aka, someone bidding 200k for above player). The fact that 2 people are bidding on a player shows he is still under value.
Also, the "fair market" value is determined by the bidders, not some calculation. If users have more money on average, the overall price will go up for players on average. The above player may sell for 40k, yet still be in fair market value. This is called supply and demand and is one of the ways day traders are able to exist.
Having a drop dead deadline will unfairly place a ceiling on the fair market value. The player will go to the last bidder, rather than the highest bidder. It would probably lead to some players reaching market value, or slightly above, a bit sooner. However, overbidding and underbidding would happen more often, rather than less.