Plenty of people these days make use of the global market offered by the Internet to get value for money on their purchases. And it is true that the freedom to buy equipment for all over the world has had a positive effect on consumer power. But it is a little bit different when it comes to home boiler parts.
This is because modern gas central heating boilers (for example) do not have many parts that you should be messing around with if you do not know what you are doing.
But the other thing that the Web has brought to the consumer is knowledge. This means that you can find out plenty about home hot water boilers (or whatever) via the Internet. Some companies have quite extensive sections of problem solving on their web sites. They often have lists of approved engineers, too.
Some home heating boiler parts are user friendly, and can be bought over the Internet at good prices. But this is not usually the case. If you can identify the problem then you can easily find out what the part costs, what kinds of parts are available (do you really need to replace the whole circuit board?). It puts you in a better position to get what you need and deserve from the professional who does the repair.