Answer:
The farmer had overworked Gulliver. And now that Gulliver has become thin and might even die anytime, it would be a better option to sell him while he still has some price on his head and will most probably make some money out of him while he can. Besides, the queen, being the more authoritative one between them, will have more control over the mere farmer. And who would want to defy the request of the queen? so, it is a great chance for the farmer to profit from the thin and sickly Gulliver while at the same time following the request of the Queen.
