Isn't policy that you don't replace or give refunds unless the items returned are complete? If I didn't return something as important as an I/O shield when returning a motherboard, I would not expect to get a refund.
Seems to go against what you're saying above.
Personally I've often been tempted by B-Grade, but lack of clarity on what is included holds me back. For example, a motherboard, I would never buy without an I/O shield (leaving an ugly hole round the back, dust/dirt getting in etc) - wouldn't care less about sata cables, manuals, drivers, box etc... but some things are important.
90 day return is a huge step in the right direction, but would not of been the area I would have put your effort into first. Either reflect the price in what you are selling, or have some form of B-Grade-Grading system. My 2p worth.