I began to wonder why it was only that gormless estate agent faced greaseball, Daniel from Daniel's City who always (eventually) bought my stuff. I began to wonder if I was invisible to other players. I began to wonder if I was overpriced. I began to wonder if I smelled...
I put it the test (not the smell) and put up a bunch of 5 measuring tapes in my Trade Depot for only 5 credits. They hung around for a few days until Daniel's City eventually bought them up.
What am I doing wrong? Why can't the world see me? what have I missed?
You are clicking the "Advertise 3h" checkbox right about the "Put on sale" command button?
Only "advertised" items are visible on the GTHQ. You can only "advertise" one item you are selling at a time (without paying additional simoleans) so it makes sense to be particular about what you select if posting multiple items. You can "advertise" another item for free again 3 minutes past your previous free posting. The advertise last for 3 hours so it is good to keep an eye and re-"advertise" if not sold within this timeframe.
Not clicking that checkbox when posting keeps items visible to only those who just happen to be visiting your city and is a way for you to sell items to your friends or at worst, Daniel.
Presently, the only way you would ever know someone visited is if they bought something.
Would be cool if they added more social features.