I'm not sure if you already have an arrangement with a LFS or not but the ones i've talked to will take on donation but nobody wants to buy. The store most of our fish came from the platys only sell for 99 cents each anyway.
As for when they'll take them as donation, I was told 2-3 months depending on size.
I just had my 3rd batch of fry & expecting a 4th possibly tomorrow. I have a 100g community tank & 3 net breeders. With my guppy & my most recent batch of platy fry I have had a gut feeling & put the mom in a net breeder. An hour later there have been babies popping out. I've waited a few hours & then taken the mom out. The 3 platy fry I found hiding in my tank right after the guppies were born were in a net with part of my guppy batch for 3 wks. They were then as big as my green neons so we felt relatively confident in letting them out. The guppies are in varying degrees of size, none quite as big as the platys so they went in our 20g fry tank.
I plan to keep my day old platy fry in net for at least a week before I move them to the fry tank. Depending on the number & growth of my next batch of guppy fry it might be about a week before I move them too.
I'm telling you all this just cause I think it might be easier for you to do like I am instead of moving the mother into the other tank prior to birth. If you do it right you don't even have to take her out of the water or anything. You just want to make sure you don't move her too soon. It's not good for them to be in the nets for more than a few days cause it stresses them & they don't seem to really eat (I put my guppy in net at least 2 or 3 times thinking she was due anytime). It can also cause them to miscarry.
Either way it's up to you. Just my bit of info & advice even though I'm relatively new to this as well. Really the best way to get the answer to your question though is ask your LFS & see what they tell you
