"Couple" meaning "pair" can be dominant hand "U" shape, palm towards signer; non-dominant hand, "C" shape, palm towards dominant hand; "U" slides down thru the "C"; "C" closes to "S" after "U" clears it.
Ugh, terrible description.
Married couple, just indicate "two-of-them", "married".
In the example sentence, "A Deaf COUPLE came into the store yesterday," first, it would depend on whether or not the signer knew the relationship between the two. Were they married, dating, friends, siblings? Two Deaf people together doesn't necessarily mean they are a "couple". :P
If you
know their relationship, or they introduce themselves, then use the relevant signs ("sweethearts", "married", "husband"-"wife", "friends", etc.).