Translation:The sales clerks are beside the tables.
I believe that "salespeople" and "salespersons" are both correct. "Persons" is a correct plural of "person", though used more commonly in formal situations.
This isn't in the accusative case. They're not being used as a direct object.
Nope. This is a purely locational statement. Even in English, "to be" doesn't ever take the accusative. A direct object is a word in which an active verb acts upon an object:
"I hit the nail (accusative) with the hammer (instrumental)"
"beside the tables" is a preposition, making "tables" the object of the preposition.