seo specialist: I'm sure you don't mean what you say in point a). It would result in the following sort of (meaningless) thing:
used sports equipment used skis used snow skis
The whole point about commas is that they "separate what needs to be separated".
Sure, it's possible to leave out spaces between phrases:
used sports equipment,used skis,used snow skis
However, you cannot possibly intend to suggest that what you have in mind is the likes of
usedsportsequipmentusedskisusedsnowskis
or even
usedsportsequipment,usedskis,usedsnowskis,
can you?!!
Duncan