This: …people calling even though they really weren’t interested in what was being sold?
More precisely, people who were interested in looking, or maybe getting detailed information, but who had no intention of buying. Typically the request for “serious inquiries” would be with an item that lots of people find intriguing, but few can afford.
I would also say it’s meant to ward off the typical stoner Craiglist buyer who says “Oh, this guy is selling his car for $40,000? I think I’ll offer him $1,000 and tell him I’ll come by to see it sometime between the next ten minutes and three weeks from now, so he should hang by the phone and wait for me to call.” Or perhaps those are just the buyers I encounter…
I think, in some cases, the phrase is also meant to give an air of seriousness about the offer itself. As in “This patch of land in the Everglades isn’t offered to just anyone; no, you have to be serious for us to consider selling it to you! It’s that exclusive!” .
It also plants a bug in the head of the potential buyer. “Of course I’m serious,” they think. “I’m a player! I buy and sell stuff like this all the time!” The last thing in their mind before they make the call or send the e-mail is, “Yes, I’m serious.” Which is a good frame of mind for your buyer to be in.
That’s why the salesman in Glengarry Glen Ross tells a supremely uninterested prospect “I know you’re serious.”
I think this is the key. If someone is trying to back out of a deal, they look for easy excuses. “I was just browsing” is the classic excuse - people who work retail sales probably hear it 100 times a day. If the ad says that you must be serious, the seller has a little more leverage. By showing up, you said you were serious; if you’re serious, you have to prove it by buying.
It’s amazing how many sales techniques depend on creating an imaginary obligation.