To sell resources, materials, or finished goods in Imperialism, click the third button in the main menu (with the "$" sign, called "Give Trade Orders") near the top of the screen then the right-hand tab in the "Orders" column unless it already says "Offer". You may offer to sell anything you make or harvest except raw food, if you are not bidding to buy any of that item.
If you offered some last turn, the slider will be at the same number or whatever lesser number you have currently in stock. If you did not, clicking the tab stub will open the slider and set it at the maximum permissible quantity (your total stock or your merchant marine space, whichever is lesser). (Note the difference from Imperialism 2, which sets it at 1.) To change the number being offered for any item, click the appropriate arrowhead in the "Quantity to Offer" column.
After clicking "End Turn", when the program has done your selling, your resulting cash balance will show at top left (giving you a rough guide to how much you should restrain your buying if you will be in debt) and you will receive other nations' selling offers (if any) then see the "Deal Book" for details of what you bought and sold and some of the other factors affecting your cash flow. You may re-inspect the Deal Book later in the turn from the "Help" menu reached from the question-mark button at top right of the map screen.
If the market price is below its original level (e.g. $900 for goods), you may be pleased to note that minor nations whom you have colonised or conquered will buy your offerings at the original price.