Having worked in the industry I stocked several spares for
SSB,
VHF,
instruments, ICs, transistors etc.etc. along with my own
instruments. After spending a few years traveling the
Carribean,
Mexico,
Venezuela,
Belize, etc, I've come to a few conclusions.
First you have to be VERY circumspect wherever you go in how you advertise your availability and expertise. There are usually local laws which prevent one from outright competing with the locals, no matter how competent or convient to the other cruisers they may or may not be and you could run afoul of the local authorities with bad consequences. If you are careful you can still insinuate yourself in and do some business.
In general, most cruising sailboaters will love you if you successfully solve their electrical/electronic problems and your reputation may spread. They, will, in general however, hate to pay you what you are worth and may even resent the fact that you charge them at all (believe it or not). Some cruisers even feel (the more
remote they are the more true this seems to be) that you OWE it to them to provide some spare part once they discover that you have it and that they NEED it to get going again and you don't need it. Again, money is another bad subject in this regard.
You other cruisers may not want to hear this but the big
power boaters cruising out there are much better at paying you what you are worth and do so with a good attitude.
The next time I
head out I am carrying only what I need for spares and will not advertise that fact. I am no longer going to be a provision source for others who often do not carry even the most basic of spares...it will surprise you! Now that does not mean that I have become a sailing cynic merely a sailing skeptic (the difference is one of positive attitude). I still gladly help others yet prefer to keep potentially negative experiences to a minimum.
Even so, recently my fiancee bragged (complained?) to some other sailers gathered onboard a gracious host vessel how many spares that I had on board. One sailer immediately asked for some S/S fasteners and, yes, I went back to my boat and pulled them out. He never even offered to give me any compensation, like a
beer or anything. Now I had to caution my fiancee to not advertise for me. I don't begrudge such things yet do feel taken advantage when someone doesn't even make an offer other than "thanks".
Be cautious, be careful, be thankful, and be happy!