You're asking simple questions , but most of the answers are complex .
The eBay question - $12k to $9k range .
As far as overseas auctions , I recommend that a person really do their homework on historical prices (both recent & distant history) , trends , demand for specific motifs , subjects/artists , sizes & mat'ls .
The demand for / trends in selling & buying paintings can be a very fickle beast , with some notable exceptions .
I've always 'done well' with overseas auctions , and limit the things I'll send over to stuff in the +$35k range .
I always try to hold off selling until I can 'strike when the iron's hot' , as best as I can predict , anyway .
Most of all for me , besides doing the complex research/forecasting , is always the credo "The deal ($$$)is made in the (original) purchase , not the sale !" .
There are certainly other spots to advertise art items besides eBay , but most o/l sites do require subscription fees & are often scoured by dealers who prefer to buy at wholesale , or below (collectors do use these resources too , though) .
If you have not made a contract with Christies , and if putting up your items on eBay or elsewhere would not jeopardize that deal ....what's the harm in trying (with a reserve price you can live with) ?
All the best to you & your pending profits !