Tony, I don't know the value of a BN1 tranny, they don't come up for sale that often, but I do know quite a bit about e-bay.
One of the things I noticed is that things generally bring more money if you set the initial bid lower rather than higher.
Case in point I was selling a TR3 speedo recently, I set the opening bid at about what I thought it was worth. Failed to sell twice, the third time I halved the opening bid, and it sold for more than the two times it failed to the first time.
I have had this happen to me many many times, generally I start low unless it is something that I really dn't want or need to sell that much and only will if you get top dollar.
You can set a reserve if you want to make sure it doesn't go too cheap, but I think this has somewhat of a chilling effect on the psychology described above--when they bd on cheap item with no reserve they start thinking of it as "theirs" and ultimately bid more and more, also more people I think tend not to bid with a reserve.
All my opinion of course, no guarantees, my low start has backfired occasionaly, but usually on low demand items.