Never used ebay as my horse is an inbetweeny size, dont usually buy saddles without having them fitted by a master saddler.
I see you are in sheffield, I live just outside it, near Holmfirth. You should go to Throstlenest saddlery, they have millions of second hand ones which are reasonably priced, plus if you take your horse there they will fit it for you.
This is there website, but if you want 2nd hand you should go down there in person. http://www.throstlenestsaddlery.co.uk/
I do use ebay a lot and over the years have fitted 4 or 5 horses with saddles from ebay, it is just a question of knowing which makes fit, which can be slightly adjusted, and what the price should be.
More recently I have converted to Saddle Company saddles, getting hold of a fitter is a bit of a pain to adjust them but I consider them to be a better fit generally than Wintec/Bates/Thorogood and a lot less risky than English leather saddles in terms of fit
You can get bargins on ebey but you can also get severly bitten. You r better off buying a saddle that you can see and touch to check that everything is as stated and its decent quality etc. Remember to factor into your buy price if you r going to get it fitted by a saddler, as your bargin on ebay could turn out quite expensive if it needs a complete reflock, new girth straps etc. Most saddlers you buy saddles from should do all this within the price as without doing the work the saddle isn't correctly fitting so they won't make the sale.
I refit a lot of SC saddles bought from ebay and you get them in varying degree of condition. One I saw a couple of months ago was virtually brand new - the flocking hadnt even bedded down yet and she got it half price. I saw one at the weekend that needed a complete reflock, na dlooked as though it needed it for some time (never seen one quite so bad) but the rest of it was fine. The others have all been variable - flocking ok for another couple of months depending on wear and the condition of the leather commensurate with the price.
Get them to send you as many pics as possible so you can see the condition of the panels, girth straps etc. But you always run the risk that the saddle wont fit unless you have had a saddler try the make and model already.