The key here is disclosure. To be safe and fair, make sure all parties have knowledge of and agree to the seller/contractor carrying out the repairs. Also be sure that the seller/contractor accepts the responsibility that will come with acting as contractor on his/her own home making repairs on behalf of the buyer, especially if there are issues with those repairs after closing.
If you do, be as specific as possible and get it all in writing and, of course, if the seller is not licensed to do a specific task, such as electrical or plumbing, that those repairs are left to a licensed professional regardless of the seller's ability to do those repairs.
Best of luck,
Barbara L Smith ABR, GRI, SFR
*5 Star Professional*
Phila Mag, Sept. 2012