Options:
1) Use PMs directly at a store to purchase items
2) Use PMs directly as a "barter" item with another person
3) Go to the local PM/Coin/Exchange dealer and get X% of spot for the current paper currency. then run out to the store and purchase goods
Most likely order of acceptance will be: #3, then #2 then #1, especially if you are talking about big chain stores. As Silver Rocket Bitches! stated, a smaller privately owned market will be far more likely to take PMs directly, than a Krogers. . .
But if I owned a big grocery chain, I'd set up a company owned assay booth in every store so the people could trade their metal for paper, and then turn around and buy stuff from my stores with the paper. That way I could increase my profits. Make ~10% on the metal exchange and another ~10% on the food selling. .