Bear in mind that it is in the jeweller's interest to perpetuate such superstitions as it means they can persuade many customers to buy a new ring instead.
Buying a diamonds as a romantic gesture, the buyer is vulnerable for two reasons:
1) They're unlikely to have much experience of buying jewellery; for many, this is the only occasion.
2) They probably feel very strongly about the gesture; it's very important that they get it right. Overspending pales beside the risk of offending the love of their life and/or subjecting them to negative judgement by their peers.
As a result, it is all too easy for jewellers to intimidate the buyer by telling them exactly what is 'expected' and charging absurd prices for it.
Still, 'real' superstition or manufactured one, we didn't fall for it. I suspect many do though. A web search for "used engagement ring back luck" reveals the superstition is pretty widespread.
[Edit: Not that I mean that many people actually _believe_ that the used status of the ring will affect their marriage - I suspect they're more concerned on how their partner and, perhaps more so, their peers will judge their decision]
After a certain age is reached though, the stigma seems to go away somewhat. We got a 'vintage' ring (about 100 years old) in the end - but they're hard to find; you won't find them in most chain jewelry stores.
Incidentally, the reason you don't see many used bands is different; the metals are worth so much now that it's more cost effective to melt them down than to resell them at the depressed price the market demands for used jewelry compared to new.