In "The Devil and Tom Walker," what characteristics of the wife suggest that she already belongs to the devil without having to bargain?

Oh, almost all of them. She embodies many sins (as does Tom in relation to her). As Irving tells us:

"He had a wife as miserly as himself; they were so miserly that they even conspired to cheat each other. Whatever the woman could lay hands on she hid away: a hen could not cackle but she was on the alert to secure the new-laid egg." 

So, she's miserly, she's mean, she nags, she conspires, she's loud, she's got a nasty temper, and she's "strong of arm," which I assume means she hits him sometimes. 

She's specifically labeled as a "termagant," which means nagging woman, but also a false deity. Taken together, she's pretty much on the dark side.