most of the time these business are just selling at market prices which pushes the price lower
Yes, some sell, some hold. Some of the people who bought items at the store are using mined coins, some are using bought coins. Some of the purchasers would have otherwise sold those coins at market, some would have held them. Retailer insta-conversion is a factor in the BTC exchange price, but the effect is greatly overstated, imo.