If ya sell them separately, you'll probably never sell the rears. Alternatively, if you do sell the fronts separately, you may get a slightly higher price due to the lower shipping costs for fronts only, (it's still going to be high though).
Personally, I would offer as a set and give the high bidder the option not to take the rears. Spell that out in the listing. If they opt not to take the rears, then put them up on ebay for $1 and shipping, then see what happens.