When breeding fish, remember, the LFS wants stuff they can sell. Pure-strain fish are more desireable than mongrels, and big fish are better than stunted little wretches. Older fish are better than newborns. Ideally, ship them off when they are about 20 mm long (about the size of a small neon). At that size, the LFS can be sure to keep them alive, mix them with other fish in his tanks, and have them big enough to attract buyers. As donna123 said, that could be 3-5 months from now.
Hence, a raising tank is a very sound investment. Instead of managing to get 2-3 babies to full size in your breeding trap or community tank, you can get 30 or more to a decent size. Even at 50p a pop, that'll be 15 quid right there, and after two or three broods, you'll have easily recouped the cost of buying a 5-10 gallon tank and its filter and heater. Filtration need only be simple (I use an air-powered box filter) and lighting is totally unnecessary.
With mollies, adding a bit of salt to the water can be very useful, and if you want to minimise your losses, I'd consider adding salt essential. Also, remember the more greens they get, the better their health and colour, and problems like constipation are avoided. Baby fish need up to 6 feedings per day, and set the heater a little higher than you would for community fish; mollies, by preferance, like it on the sultry side. 28 C will do nicely, and that'll pep up their immune system while speeding up their metabolism, both essential with young fish. When fish are stunted or sick as babies, even as adults they are often smaller and less attractive than otherwise would be the case, however well cared for subsequently.
Cheers,
Neale