you cant get a divider for a male and female. you'll need separate tanks. As for selling her it probably wont work. From what I can pick up you've had her for a while. No breeder is going to want an older girl. For a pet you'd have to find the right person and then they will have to be willing to pay your price for her. I don't know where you planned on selling her but it is pretty irresponsible just to get rid of her because she doesn't fit into your tank anymore .
Feel free to message me any questions, I'll try to help the best I can!
Rummy- Mini Rex Rabbit
Diz jr- Betta
Yoshi- Crested Gecko