to buy or sell? either way get a fitter, they usually have second hand saddles as well as new, so you can get a good saddle that fits, but doesnt have to cost the earth.
I know one person who decided a saddle fitter would cost too much, and so ended up buying 7 saddles on ebay, and selling 6 on ebay before she found one that she (not a saddle fitter) thought was correct.
It cost her a fortune in time and money, because she didnt get back what she spent on several of the saddles.
TBH I (Im not a saddle fitter!) dont think that the saddle she has settled for is a very good fit as it rolls across the back!
Saddles direct are quite a good deal. You can get three to try and its £20 postage. Worst case you have to send all three back its just another £20. Or eBay if you know what you are looking for and get a bargain you could sell easily if its not suitable when it comes .