1. Bring a small saucepan of water to a boil. Add scallops and cook until firm, opaque and just cooked through (1 minute). Drain and rinse under cold water until cool.
2. Slice just enough off the bottom and top of the grapefruit to stand it safely on a cutting board. Remove the peel and pith. Hold the fruit over a medium bowl and cut between the membranes to release individual grapefruit sections into the bowl, collecting any juice as well. Remove seeds. Transfer the grapefruit sections to a serving bowl, reserving the juice for the dressing.
3. Whisk shallot, vinegar, mustard, and oil into the grapefruit juice. Add the scallops and crab to the dressing. Toss to coat.
4. Toss together grapefruit, lettuce, tomatoes, and avocado. Toss gently with the seafood and dressing to serve.