Will the stores always sell the bitcoin they receive right away?
How do customers get bitcoin they use to give to the store?
Most of the big companies who accept BTC will immediately convert to fiat so there is no risk (or upside).
I would say most of the companies accepting BTC do so as a gimmick and have no intent to hold on to it.
overstock do a $1mill a month in bitcoin.. and guess what, they only value the bitcoin in dollar. but never actually convert it to dollar. they are hoarding.
secondly bitpay do not throw bitcoins onto exchanges to sell for dollar. they just use the dollar value of exchanges for establishing prices. but the whole btc-fiat. is done privately through private investment/capitol funds.
its only coinbase that are swapping the coin on an exchange that is being measured by the community. so relax about companies dealing with bitpay. but your half right in regards to companies using coinbase.... well maybe a 3rd right. as even in coinbases case not all merchants sell 100% back to fiat.