The website www.Craigslist.org has been credited — or blamed — with starting the online advertising revolution that has decimated the classified advertising sections of newspapers large and small across the country. 

For decades, classifieds were the most profitable part of the newspaper business. With incredibly low overhead, they didn’t require salespeople to call on prospective clients — calls came into them.

Someone wanting to sell their car, old bedroom sets or find a new employee would write out 20-30 words, call a number and place the ad. Sometimes, they’d enhance the listing with a special font or bold type, but that was about it. It cost around $20 and was a huge moneymaker for the papers.

According to several studies I’ve seen, newspapers generated 40 percent of their revenue and much of their profits — billions of dollars industrywide — from classified print advertising as recently as 2000. Today, that percentage is far smaller, but it still represents a significant, if shrinking, source of cash.

Craigslist, the website started by San Francisco resident Craig Newmark in 1995 as a private email list distributed among friends, has ballooned into a one-stop shop for everything from knickknacks to apparently, accessories for ductwork fabrication. And unlike classifieds, most listings are free.

As part of my job, I receive regular alerts through search engines like Google whenever news or posts containing words or phrases such as “ductwork” or “HVAC” show up in the sites they monitor. On occasion, I receive notifications that a seller on a local Craigslist site is listing ductwork or other sheet metal products for sale.

Where does this stuff come from?

I’ve never contacted any of the sellers, and I wonder where they got these materials. Were they excess inventory or left over from a job? Were they stolen? Did they come from a sheet metal shop? Does the shop owner know these items are available?

The practice of some HVAC technicians to moonlight their professional services installing furnaces for friends, family and sometimes strangers on weekends has been controversial in the HVAC industry for years, although it’s not a new phenomenon. Websites such as Amazon.com have long sold complete systems direct to consumers without the requirement to pay for professional installation.

I can understand why some homeowners would be willing to take a chance by purchasing an air conditioner or furnace on Craigslist or other websites even if they didn’t have someone hired to install it. Many consumers are solely motivated by price.

But buying duct boots, reducers and collars of various sizes — all items I found on Craigslist while preparing to write this column? I can’t imagine many homeowners going to a website to buy those, and especially not a free-for-all site such as Craigslist. The listings use industry terms that would not be understood by anyone without at least some knowledge of sheet metal work.

I find it hard to believe that very many contractors would decide to check out the online equivalent of a rummage sale if they needed a few parts. But plenty of people with access to ductwork seem to think it’s a good idea, based on the listings I found. 

What do you think of the practice? Do you ever buy sheet metal accessories from unconventional sellers?