Speak to a "Seneschal" NPC in any cite scene which gives u a trading licence in that city. Best to get the biggest as it's relatively cheap at 20,000 Denars for 480 ingame days, about 20 IRL days letting you store 100,000 items on the market in 10 different slots.
Sell miscellaneous item at Artisan. Best to sell items from Wildlands and Frostlands PvP zones to players, higher tier and some high demand weapons sell for hundreds of thousands sometimes, checking the Zeonica market before selling directly to players can avoid you getting scammed.
Items will sell easiest in Zeo as it's the trading hub of the server, but places like Baltakand are sometimes good to sell at as they're popular spots for players going Frostlands. Sometime items can be bought cheap at one and sold at the other, or other locations around the map such as capital cities that have warehouses like Omor.