In a large shallow glass or ceramic baking dish, cover the crabs with the buttermilk and refrigerate for at least 1 and up to 4 hours.
On a large plate, blend the cracker crumbs with 1/2 teaspoon each of salt and pepper. Alternatively, season the cornmeal with 1 teaspoon salt and 1/2 teaspoon pepper.
In a large skillet, heat the oil over high heat until shimmering. Meanwhile, remove the crabs from the buttermilk and shake off the excess. Add the butter to the oil and, when it stops sizzling, dredge half of the crabs at a time in the crumbs or cornmeal. Set them in the skillet, undersides up, and fry, turning once, until cooked through and crisp, about 4 minutes per side; adjust the heat if the oil gets too smoky.
Set a crab on each bun, top with the tomatoes and Curried Pepper Relish and serve immediately, with lemon wedges on the side.