Although this would be a great addition for buyers, I think this is not really viable, as most sellers are accepting a variety of payment methods [which generally include RS GP [which is a fluctuating, virtual currency]] and otherwise most of the sellers don't have an Auto-Buy price, but rather are looking for an auction style bidding.
On another note, if someone wanted to update their price, they'd have to ask a mod to change it [even if such a feature were to be implemented], because normal users can't have mod powers [which I'm sure would be one].