Here at CP we have a "contract" that says that we will always take our cats back into care if there is a problem or change of circumstace. If we found out that someone hadn't neutered their kitten and were planning to sell her on the free ads, then we would be delighted to get a call from someone responsible who had stepped in! We would probably come and visit you to do a home check and if we felt it was unsuitable, then we would have the "right" to take her away and re-home her, but we would spay her before she was re-homed again.
If we felt your house was suitable and you were willing to keep her then we would check that you had neutered her and then leave you to it!
I, of course, do not know what the RSPCA would do, but I would definately step in for the meantime and then deal with the issues later. The main thing is to keep the kitten off the street and off the damn free ads.
On a side note, people often complain about how picky CP are about who are and where our kittens are adopted, but (generally) we don't have people handing them back. Perhaps if certain rescues (not just the RSPCA) didn't hand out kittens and cats like sweets, people like you wouldn't have to step in and stress themselves silly *rant over* Sorry!