I make my own biodiesel by collecting and processing it myself. As of today my cost per gallon not including labor (of course) is approximately $0.85 / gal.

I'm not saying that fellow is ripping you off but that does seem a bit unusual to charge so much.

Just for giggles you might ask him if, "he's current on all his Federal or local tax documents as a fuel producer?"  LOL

Anyway your Yanmar should run just fine on quality bio (this is not a recomendation, check with Yanmar) all of my diesel vehicles and all except my really old tractors do too.

Best to you and like someone just posted, read up a bit on it, lots of info out there.

Anyhow, the benefits.  First off, biodiesel is much closer to being carbon neutral.  The carbon in the fuel came from our atmosphere and was fixed by the plants that made the oil.  That carbon will go back into the atmosphere of course.  But since it was fixed such a short time ago, its a wash.  This is very different from liberating additional new carbon from petroleum.  Of course too, the re-use of the veggie oil is a nice benefit.  Also, the economy is stimulated locally instead of remotely (i.e. you are putting your dollars into the hands of someone who will likely spend them in your area rather than shipping the money to the Mid East or Argentina or wherever the crude came from to make your regular diesel.

On a more practical side, biodiesel will keep your engine a lot cleaner. This is true for at least two reasons.  First, it actually acts sort of like a detergent to dissolve crud.  It does this better than the additives that go into regular diesel.  Secondly, it burns much more cleanly. For example, there is virutally no sulphur in biodiesel and particulate emissions are lower.  The scrubbing effect can, however, be a double edged sword initially.  Moving staight to B-100, pure biodiesel, right out of the gate can and sometimes does scrub your engine so effectively that it leads to clogged fuel and/or oil filters.  Once the engine and fuel system is clean, they will actually clog up less frequently than they otherwise would.  But, it is something to keep in mind initially.

The second consideration for biodiesel you won't have to worry about at all.  That is, biodiesel will gel (i.e. stop flowing) at a much warmer temperature than regular diesel.  In areas where it is consistenly cold (i.e. below freezing), you have to be careful to mix bio with regular to keep that from happening.  But in the tropics, that won't be one of your issues.

The amount of fuel used by this engine is very small, given its limited operational cycle.  But one use you didn't mention is emergency propulsion - like when you're being blown onto the rocks and can't tack back upwind.

straight veggie also can cause stuck piston rings, the glycerin doesn't fully combust in a partly loaded engine and ends up caking up the rings till you have enough blowby to burn the top ring and then you are set for an overhaul.

the only way i would use straight veggie is in a properly loaded engine, then i would start on diesel till warmed up, switch to veggie for the run, and then shut down on diesel.

your little engine is likely an indirect injection engine, which is a plus
idi engines tolerate alternate fuels better than di engines.

i think you better research this out well before you switch over, otherwise you might very well have some expensive repairs, or be left stranded out at sea?

As others have said the Bio-D will clean out your lines, your fuel tank and injectors when you first start to use it, so have plenty of fuel filter handy :-)
The price he's asking may be do to the paid fuel taxes, maybe not. Ask for quality check cert. Best places will have something similar to what Europe has, Germany and France I believe has the hardest testing.

There is also a quick way to tell if the Bio-D he's selling has been done completely and with quality.
Take a small amount, say 500mL, add about 70mL of plain tap water. Give it good shake, water should settle to bottom in about 1/2 hour, if it seems to have little bubbles suspended in the fuel , then washing was not done completely and you'll have nasty sludge problems that will infect your fuel tank, which then would cost you more to steam clean out.
I would say to talk with the seller, ask way so expensive, if seems reasonable, probably due to higher cost of Methanol. Large tanks here have risen 5x original.

Then if you do go with Bio-D begin by adding to Dino-D 1/4th mix, this I still do even with temps around TDC <-Two D(*&^ COLD!!