Items you put up for sale are available at merchants throughout that land.
There is a trick some people consider an exploit -although CSRs have said it isnt- where you sell your looted mats or dug mats to the wandering hawkers throughout the wilderness, then just retreive them from the shops once you get back to town, easy teleport-to-town for your loot..
You can retrieve your own items up for sale from any merchant in the land - and see those items in/from other lands you have for sale there (-but cannot do anything with them -)
For other players to see and buy what you have up for sale, they would have to be at the correct merchant, eg raw materials for mats, Heavy Armour merchant for HA, and the merchant would have to be af an adequate level.. mainland shops have level caps on what they sell, so the merchants nearest where you start might sell gear up to lvl 50, further away in the main city they sell up to q100, and you have to go to the outlying towns to buy lvl 150 up to 250 gear. The level caps are maximums, not minimums, so you can always see ALL the lower level stuff available, which is why it is highly recommended to use your filters