TBH, 99% of the time programmers shouldn't do it, it's a very special case. I regret we did the eCommerce products through catalogues, if doing it again we'd use them for custom fields but otherwise implement it as a totally new content type. That's a close-thing though. Classifieds is kind of bolted on, doing validation/devalidation behind-the-scenes so is a special case.

We could do a worked example, but there are so many addons nowadays, we're swimming in examples. Probably for v10 I'd just advise people to look at the iotds addon (which is no longer going to be a bundled addon, so you could very easily download the tar and examine all the different hooks etc in there).