Swap meets are events when sailors will gather and sell items they have and no longer need. The prices are good, and sometimes you can trade. There are a lot of these events in places where foreigners on yachts tend to gather. We attended swap meets in La Paz (mexico), Vava'u (tonga) and soon we'll go to the one in Whangarei.
If you live in a big city with lots of sailors and marinas, chances are there will be a good used marine goods shop around. Many cruisers opt to sell their gear there or to put their items on consignment (thusly eliminating the need to actively search for buyers).
If you know what you are looking for, you'll most likely find it, or you'll find something very near to it. Second hand shops are full of bins consisting of cryptic metal parts and angry tangles of wire, the quantity and diversity of it all can be quite intimidating. We have often left more confused than we started, it's for this reason that you need to enter stores like this with a plan.
Stores like this will often sell used sails too, even sail shops sometimes have some pretty sweet deals.
Examples: Stanley Marine in Whangarei, Blue Pelican in San Francisco, Minney's yacht surplus in Costa Mesa etc.
*Note that all of these places have big yacht concentrations. The more boats there are, the more used gear you will find.
It's a good idea to look at marina notice boards when foreign yachts start populating your area. We've seen many ads with amazing deals: lots of cheap dinghies, outboards, paddle boards etc. More often than not, people are just passing through and want to get rid of items quickly.
We walk by the marina office everyday, and always stop to look for new notices. This is how we got our extra solar panels. We purchased two 90W solar panels for 150$ from an older English couple, they sold these panels to upgrade to larger models (they have more electronics and toys than we do). We've installed them already and they work perfectly, the owner of the boat even came to help us set them up (the sailing community is awesome like that).
Every time we bring recycling to the marina bins, we look at the trash to see if there are any goodies. The other day we found a working 600W inverter, with american-style outlets. Chances are the owner wanted to upgrade to a higher wattage, and didn't think it worth while to sell this one (given the voltage). There aren't that made north american yachts passing through NZ on a yearly basis, and the ones that are there are not necessarily looking for inverters (most yachts need bigger inverters than this). This was a godsend for us, we needed to upgrade our little 250W. Now we can charge both of our laptops at once (yay).
If you keep your boat near a big city where lots of racers congregate, chances are the marinas in the area will have bins with lots of good gear. Racing yachts are particular about their boats, some count the number of tacks they make and replace the sail when they reach a certain number. I imagine the sail's performance may be reduced, but to a cruiser or tinkerer, there are many things one can do with an old sail. Friends of ours have found great-looking sails in trash bins around big marinas in Auckland.
We've seen entire boat ovens, rigging wire and jabsco toilets in bins. While these items may not be working, sometimes you can salvage parts. We've collected many items over the past 2 years, we almost always find a use for them*.
*Note: in no way do we want to become hoarders, it's important to be selective in your findings. Otherwise, after a year of accumulation, it's always a good idea to review what you have and to get rid of stuff. Swap meets are great for that.
Back when we lived on land, we moved a lot. Instead of purchasing new items, we would get them at thrift stores and then return the items when they were no longer needed. In Montreal, we purchased items from Les Petits freres, a store who's proceeds goes into helping the elderly. They could help themselves to items in the store for free. The items you give and take serve a much greater purpose than you think.
Recently, Devine & I wanted to buy a sewing machine so we could do some projects on the boat. Most of the machines we saw in store didn't fit our budget. There is also the fact that we have limited knowledge of such things, and turned to our sailing friends for model suggestions.
Most of the people we know carry a sailrite, a heavy-duty sewing machine (that weighs as much as we do) with which you can do sail and canvas repair. Buying a new sailrite will cost you anywhere from 650-1300$US dollars, if you consider the cost of sail repair it is a good deal (but not one we can afford).
We settled on a Toyota sewing machine for 35$, bought in a second-hand shop. With it we can do repairs to canvas and cushions, not certain if it'll work for sail repair but we'll give it a shot. Otherwise, nearly every north american sailor carries a sailrite, we'll make friends and ask to borrow theirs. We did this while in San Francisco, and repaired our mainsail cover with it. It never hurts to ask, and sharing is nice isn't it?