That's a hard sell. If it's just $300 why not take care of it yourself and not have to make explain why the tranny slips. Potential buyers might fear a new tranny or overhaul is due. If I were you Id scan for codes and change the tranny fluid and see if that solves it.
I have scanned for codes and there is nothing and it has never had a tranny code. I was planning on having it detailed serviced then put it on auto trader. Just thought I would try this first here. I have a new truck in the driveway and need to get this one to a new home.
So I had the trans serviced at a reputable shop here in Long Beach (Stu's Transmissions) Performed a complete service and fluid flush. They reported the trans is in perfect condition. I have not felt what I thought was a trans slip after the service. The shop said it could have been something in the trans malfunctioning but was very unlikely.