Linn's Trading Posthorn?

Can someone with more experience than I please explain what the ads in Linn's "Trading Posthorn" section are? They appear to be simple trades... send 100 stamps, get 100.

Are there certain rules of etiquette to follow? Is it worth the effort? I'd like to know more about this style of trading, as people's mixtures sell for a lot more than they did years ago, and I am on a tight budget right now.

Can anyone share some info?

I also would like to know about this ... hopefully, we have some traders out there!

I traded there back when there was a half a page of ads on the Posthorn..before the internet...if a trade sounds interesting you have to write to the person who placed the ad and work out terms..it would be a lot easier if they provided a e mail address..but very few do !!