From selling other items... not AG... I would say Mid to late October or Late November after the movie.

Alot of people get on eBay to find a great deal for their Halloween costume and check other things out while they are there to get a head start on their Christmas shopping. The general hype about the movie should be rolling by then, making her an item of interest, especially if you focus on the PM aspect and that now even her dress is retired.

The other good time would be right after the movie is released. It's the Christmas crunch season and little (and big ) girls are going to be talking nothing but dolls and especially Felicity and Elizabeth. People will get online to try and find a bargin, and not realizing there was an older version, come across PM Felicity being sold. Again, the focus here would be giving lots of information about the fact that she is a retired version. Generally then, the collector in people kicks in. Especially in the midst of the movie and Christmas blitz.

But you are right... timing is important! Good Luck!