I've purchased three saddles online, had good experiences with all.
Saddle 1 was an all purpose, black leather English saddle. No brand, was one of those saddle sets.
Saddle 2 was a brown leather jumping saddle. "Stunni" brand - a cheapie made-in-India treeless.
Saddle 3 was a black leather/synthetic Ghost Dressage saddle.
Had great experiences with all three purchases. The first two buys were from eBay
, the third was from a distributor.
The first two I wasn't terribly worried about because I purchased under eBay and had a 10 day return period. The third I was worried about because it cost significantly more money but I had a 10 day trial period on it as well.
My suggestion is to be sure there's some kind of trial period. Try to get a demo or see if you can ride in someone else's saddle that's the same make/model. It's more challenging with the treed saddles because of the problems with fit (my last two saddle purchases were treeless). Try to stick with known brands. I was very lucky with my two no name purchases, but know MANY MANY stories of people who had terrible fortune with both named and no name saddles.