Offer / counter-offer? Audiogon rules

This thread gets me to thinking...I'm a licensed Realtor. Let's say I list a house for a client for $500,000 and we receive an offer for $450,000. We could counter, for example, at $480,000. Here's the rub: while the buyer is mulling over our counter-offer, we receive an offer for full price. Legally, however, the first buyer has the prerogative to accept our counter despite our having a second buyer willing to pay more. In my practice, I don't counter: I invite the buyer to make a new offer just for that possible scenario. If you counter on Audiogon, does the Buyer have 48 hours to accept or decline? 

the 48 hour rule is the reason I also never do counter offers...

One of the problems here is the obvious disagreement over VALUE.
In my completely unfounded opinion (CUO), the main weakness with bargaining is the implicit admission by both parties that 1) they don’t know how much it’s really worth and 2) they’re both out to screw every last penny out of the other guy. Some people really enjoy it, but most people don’t CUO. Some sellers must be feeling the squeeze by now but I guess their audio slush funds are tied to their stock portfolio and they can’t get off the pony. Used prices are too high. Half of MSRP is really a high price if it’s out of warranty.
Seems like that’s become the new “used list price.” Great for audio consultants in the dealer network who have used a new piece (on accommodation) for effectively no cost. 
So if your big power amp retailed for $2000, it should sell used for a maximum of $1000 for a #10 condition. Then, technological age and condition are the only problems. 

@denverfred 

One of the problems here is the obvious disagreement over VALUE.

Not sure why that is a problem.  The market will sort it out.  Buyers will pay what something is worth to them and sellers will adjust their pricing based on their level of desire to sell and their willingness to wait for a buyer who will pay their price.  That is the definition of free enterprise.

there's a distinction between an unofficial counter offer and an Audiogon rules governed official offer