Anyone can sue you for food poisoning (ecoli, etc.) but I think whether someone can prevail in a lawsuit will depend on what type of regulations apply to you and whether you have complied with those regulations.
I believe to sell your butchered chickens across state lines, you must have your birds butchered at a federally inspected plant. When you are selling small batches, it will be difficult to find such plants.
At least in Wisconsin and Minnesota, to sell more than 1,000 birds per year or to restaurants, you must use a state-inspected plant. If you sell no more than 1,000 chickens, you can butcher them at home. Exempt, or uninspected, poultry plants can also butcher no more than 1,000 birds for sale or home use.
Generally in Wisconsin, birds processed at home or at an exempt plant must be labeled not inspected, exempt 97.42 with the name and address of the producer. In Wisconsin, you can sell live chickens to customers, but these birds cannot be butchered on your farm. The buyer has to take the chickens home to butcher them.
What this means is that if you sell less than 1,000 and it's "not inspected and exempt" then the buyer is basically buying with more risk knowing that the bird was not processed in a government inspected plant. The risk shifts more to the buyer than to the seller.
This is what generally applies in Wisconsin and Minnesota, but I would guess many other states would have similar rules and regulations.