Have you ever withheld info about an item?

[steve B]

To me it depends on a few things.

General info about something I usually share, and stuff about things with limited appeal like 91 Topps or that sort of thing. I'm happier to have someone else that collects the weird stuff.

And I'll generally answer a direct question about something if I know the answer.

But about stuff that's for sale? Nope.
I find stuff, especially stamps pretty regularly where a valuable variety isn't recognized, Or ones where the seller just hasn't done any research whatsoever.
I may have an occasional twinge of regret if it's not a dealer, but looking stuff up is so easy these days that's as far as it goes.

The exception is dealers whose shops I go to. I'll share info with them. But the stamp guy I go to usually responds with "That's interesting, you should buy it." He's told me that as long as he makes his money he's at a minimum ok with someone else making a bit more.

He had a postcard by Mucha that he got from another dealer fairly expensive but not familiar. Sold to a customer who collected Mucha postcards, on a hey I found this for you sort of offer. Paid like 50, got 500. Collector turns around and sells it to a big dealer for 1000. Big dealer sells it internationally for 5000 and it ends up in a major European auction and sells for 10,000.
Turns out it was previously unknown and a pretty big deal for people who collect those.
Happiness all around as everyone made money.