I think I'd rather see players listed for what the owner would accept for selling them, so that the odd time during the season when the servers glitch but not enough to trigger a full failure/postponement, we won't see the complaints of "my $X million player only sold for a fraction of the value". Unfortunately, that has happened and will happen again.
I do think, however, that "week-trading" is a problem, at least when it's done not by managers who find players truly at a bargain, but instead just buy cheap players and list them at high prices. My idea on that has always been that for non-draftees, the maximum initial listing price would be based on how long the player has been with your team. If it's less than a month, maybe force a $1 initial listing, and scale it up so that after a season, maybe, you can list him at the price you paid for him, and then uncap it at some point later. The amounts and timeframes are an implementation detail, though, and I haven't too thoroughly considered those.
This would still allow those who actually find players who are undervalued on the market to still make a profit (as the player they paid $100k for but is worth $300k will likely make that amount in most cases), but for people trying profiteering on getting people to overpay for potential on players for whom it will never matter, you'd have to hope that multiple fish bite on the same hook.