As a rule, since I'm a moderator here and formerly worked for an e-tailer, I will (almost) never recommend for or against any legitimate retailer (ie, company that is a known name and has a good reputation--if we've never heard of it or know of it to be a fly-by-night or something, that's another story) because I see it as a conflict of interest and my "job" here is to provide general information and assistance w/o any bias.
That said, Diamond Review HAS vetted a number of diamond e-tailers as trustworthy (you're going to get what you pay for and good service), reliable (you're going to get it in a reasonable amount of time, barring any crazy problems with a custom order once in a blue moon) and with a fair return policy (you should have a reasonable amount of time to view your item and return it for full refund--maybe minus shipping, but no restocking--if you're not happy with it). You will find their stones listed in the Diamond Finder tool at the top of the page. Again, we're not selling or even outright recommending them, or anything, but it is a good resource if nothing else. We're happy if you do toss them your business, but we never pressure or say "here's a stone like what you're looking for--buy it!" or anything like that. Also, some of the e-tailers we've vetted offer some discounts (see the Coupons tab @ top of page) if you DO happen to find a stone in their inventory that you like.
Everyone is going to have a different experience with any e-tailer, same as they would with any brick-and-mortar store. You just need to find a company and/or salesperson that you are comfortable with and trust them to be able to answer your questions to your satisfaction. I'll say though, of course, everyone is going to have one area of technical knowledge different from another, so try to talk to someone who is pretty familiar with selling Cushions (as opposed to someone/some company that really only sells Rounds or Princess cuts all day long and can't tell you much about Cushions), so that they can better understand what you're looking for, the look you're going for, and can better act as your advocate in your search.
And, just b/c you mentioned that you're also looking on ebay--I know they have buyer protections and so does paypal if you pay for things that route--just be sure to check out any seller's reputation (and google them, etc. DUE DILIGENCE!) and be careful. Make sure you're only buying certified stones so that you know that you're actually getting what you're supposed to be getting, not just what some Tom, Dick, or Harry wants to call something, ya know?
Anyway...I know that was a bit of a roundabout answer, but hopefully that helps a little! When you do find a diamond (or ring, if you find a finished piece or whatever), you're more than welcome to come back, ask questions about it/them, post pictures, copies of certs, etc, and we'll be happy to give you unbiased info and opinions on them to help you get the best thing you can