Depending on the browser. Some require you to drag, some, like Firefox, require you to click.Once you are there, just drag items from your inventory to the store inventory, and it will ask you if you want to sell.
There's also a signpost button in every store in every town, or you can schedule sleep. If it's your town, it's advisable that you sleep anyway when selling a lot of items and sitting idly in town with cash.
One thing though, be careful what you sell. Don't sell products you need for future Quests. There's also no reason to ever sell anything unless you need the cash right away. Unneeded items and products are your savings. Also The West Insider can tell you what items you can sell, but the quest advice above takes precedence over TWI's advice.