And Miru is the first, because the others were deleted.
Technically he declared interest on an auction and not a firm price. Per AmiBay rules you should be first but a mod would need to chime in
This is one possible interpretation, but on the other hand he said "I'll say 400 USD as a starting
asking price" which means the
asking price is subject to change (by the seller, obviously).
In an auction the asking price is only set once and doesn't change by the seller but due to potential buyers' bids.
On the other hand, the seller changing the asking price is perfectly inline with Amibay rules, e.g. if the item doesn't seem to gather any interest (technically the seller doesn't even have to lower the price, he might increase it as long as no interest has been declared at the previous price).
If he'd posted "I'm gonna ask 400 USD as a starting price" I'd be 100% with you on this, the auction implication would be clear, but the way he worded it he gets the benefit of the doubt so since he didn't "incriminate" himself by accepting higher bids, I'll agree with Davideo that Miru's declaration is valid.
My objection is that the price was set in a subsequent post and not reflected in the 1st post. This way, a rules-following member will not know that a price has been set if they don't skim through the thread, and will miss their chance at declaring interest.