Today I bought one pound of fresh scallops from the seafood counter at the local market, but my dinner plans have changed - I am no longer planning to cook the scallops this evening and I don't necessarily want to freeze them. How long will they remain fresh in my refrigerator?
No more than 72 hours, maximum. You don't know how long they've been out of the water. I would freeze or cook them no later than tomorrow evening.
The most important point is you bought them 'fresh'.
It is my understanding that scallops have to be flash-frozen within minutes of being caught, (probably somewhere like in the middle of the Bay of Biscay.)
Therefore, any scallops you see being sold in an unfrozen state have already started decomposing.
If you don't eat them immediately they should be used instead for enriching the soil in your garden, or perhaps as 'chum' on a fishing expedition.
Re-freezing is, of course, out of the question.