Out of eBuyer, Scan and OcUK, I find that any one of the three can be cheaper than the other two.
I personally haven't had any issues with returning products to Scan and OcUK, I haven't had to retun anything to eBuyer yet. Maybe I've been lucky
Returns:
Scan - 9800pro broke died after a week or two. They happily refunded me.
OcUK - Took back a broken XP1600+ I think it was.