Taking off the lower unit doesn't give you a better view of the shift shaft. You can only get a peek at a tiny section of it rigt below the where the lower mount bolts are on the midsection.
Ebay would be a crap shoot and you could get stuck with another one that leaks. I'd change the oil and run it for a trip or two and then see how much water you get. Maybe you can live with it with more frequent oil changes?
If you were willing to DIY I would try just doing the seal under the water pumo first and se if that helps.
For $800 you wouldn't be getting an OEM Yamaha but more likely a new clone by Sterndrive Eng.
http://www.sterndrive.cc/Merchant2/m...ory_Code=SE416
They have made replacement Merc Alphas for a while and now have outboard lower units.