The market for used BMX parts belongs mostly to kids. Kids with sometimes limited budgets. Best to group related things into packages as Snazzy says, but keep everything else separate to keep individual prices down. You can always combine things for a shipping discount.

Example: You have a bar and stem. Most guys are looking for one or the other, not both. You may only get $20 because someone wants one or the other. But others may be willing to pay $15 for each item by itself.