I have found a couple of answers in an essay written by one Geoff Chapman in 1999. Mr. Chapman is an amateur of Jane Austen from Melbourne (Victoria, not Flordia). If he isn't already involved, I think he might make a useful addition to this community. He has the following points on the subject of next presentation, culled from early 20th century texts on English real property law.

1. The sale of the advowson (right of presentation) to a vacant living would have been regarded as simony, which is contrary to ecclesiastical law.

2. A statute of Queen Anne's time prohibited an individual from purchasing the right of next presentation for his own benefit but did not prohibit the purchase of next presentation to be bestowed on others.

The distinction between selling the advowson to a vacant living and selling the reversion to the advowson of an occupied living strikes me as a little hypocritical but pretty human. Further enlightenment is eagerly awaited.

Chapman's article also has an interesting discussion of the law and economics of tithes, and his site contains another interesting article on entail and the techniques of strict settlements.