Find engagement rings online

Ask for the story behind the diamond .  It's possible that you may be getting the deal of the century if someone needs the money or the ring is from a broken engagement.  Otherwise, it's unlikely that diamonds are being drastically reducded.

Stick with a trusted seller .  If you chance things with a new seller, you run the risk of being ripped off.  On some items, the risk is worth it but probably not with a diamond engagement ring. If a merchant has a brick & mortar store, stop by & check out the ring.  You may be able to get a better deal online but at least you'll know what you are buying.

Make sure that you have a money back guarantee .

Ask if the ring is sizable .

Check out traditional jewelry stores .  This will give you a feel for how big the carat weight will look on the ring, & your favorite choice of cut.

If you buy an engagement ring at a traditional retailer they will often throw in extras like free ring cleaning or ring resizing.  They often ask you to bring the ring in every few months to make sure that the mounting is secure.  These are services that you will probably have to forgo online.  See if they are available elsewhere.

Take the ring to an appraiser before you decide to keep it.  Make sure that it's what you expected & worth what you paid for it.

Don't give into impulse buys .  Be prepared well in advance for any online auction.  It's a big decision so give yourself a few hours to think it over & set your price limit.  There is nothing worse than finding out that the ring is so cheap because it's ugly.

Be prepared for shocked reactions .  Your fiancée may or may not be ok with the fact that you bought her ring online.  It's a story that she will have to tell every time someone looks at her hand.  The ring may seem less legitimate to some people.