It’s been a few years since I’ve sold any babies, so things could have changed (and also dependant on your local area), but I would list them for sale after 3 meals, but they often didn’t actually sell until they were at 10+ meals. Especially if you’ll only be selling locally, they likely won’t sell super quickly, so hopefully you’ll still have room for them if they take a few months to sell.
Edited to add that I just checked the records for a clutch I hatched in 2014. Of the 10 babies, they sold between 4 and 19 feedings, with the average being a little over 11 feeds before leaving me. For some of them, the number is pretty high because they had to wait until appropriate temperatures before shipping.