Ah, there's the rub. There are two closings, so two sets of closing costs. Being in the middle, you act as buyer on one transaction and seller on the other. So, you pay one set of buyer costs and one set of seller costs.
Its not quite that bad because not all fees apply in all cases. You won't do your own appraisal. You won't have any loan origination fees or realtor fees. You may also be able to avoid title insurance if the company will let it just pass you to the end buyer. Some of the other fees, like the title companies charges for paperwork and recording fees, and going to have to get paid twice.
This is a good topic of discussion with the title company.
Jon