Kamadan seems to be the hottest buy/sell town, followed closely by Lion's Arch. Kaineng Center is a distant 3rd, being used mostly by people who only have Factions.
It used to be that most of the buy/sell activity was in the first 1 or 2 districts of these towns, but now that the Search function has been added to the Party box, things are more spread out. The buyer and/or seller can be in any district.
So, keeping the above in mind...... It used to be that people would do stuff like "WTS >>>>>>>>>> perfect omg! special sword <<<<<<<<<" and spam it several times. Nowadays, all that would show up in the Search window would be "WTS >>>>>>>>>> per" - which tells no-one anything.
My advice would be - when you are trying to sell something, keep the Search box open and try to format your WTS so that the main features of the item are within the limited space in the Search box. For example "WTS req9 Gothic Sword, 20/20, h+30, inscribable, pm offers." would show up in the Search box as "WTS req9 Gothic Sword, 20/20" (or so) - but anyone, in any district, looking at the search box would know you have a Gothic Sword for sale and would whisper to you if they are interested.
Conversely, don't be annoyed if someone PMs you asking something that was in your original WTS, they may be in another district and can't see Trade chat in your district.
In your particular case, don't stand around in Kamadan spamming "WTS lots of weapon upgrades" Most people are looking for a particular upgrade and will try the guy who is selling the particular one first. Conversely, you could expect to answer a lot of "do you have such&such?" PMs. Try to get a few of the hottest selling ones into the Search box ("WTS 20/20 & +30 for axe" - for example) and don't change it too often, but often enough.
Personally, I don't even have trade talk turned on any more, whether I'm buying
or selling, because there's too much spam. I just use the Search.
Of course, even easier than standing around in a town, you can use the buy/sell (and price check, for reference) forums or Auction system here on Guru to sell things.