Do quests and sell the items you get from the quests and battles, or have someone take you to Halfang Bristlecrown in Vestrilund, Wintertusk and farm with you.
At level 28 you don't have access to Wintertusk to farm yet, so I'd say continue on your quests normally. I'd assume you're in Marleybone, and as you earn more and more loot to sell from battles/tasks, the prices do start to increase steeply and you can profit more and more.
I'm sorry, but there really isn't a very easy way to get gold at your level, you just need to sell your drops and quest onward The higher your world/area the better the drops are sold for.
By the time you get to level 50+ though, there are lots of bosses in Wintertusk (the area past Grizzleheim) like Erling The Unready and the infamous Halfang that give you drops with an insane sell price at the bazaar.
I wouldn't recommend farming however,because by the time you get to around level 80, most of the problems revolving around money is how to spend it.
If you are seriously convinced on getting cash though, farming stinkweed or better plants is a good source for your level, albeit slow.
Try selling items you don't need, questing, buying gold with crowns, or fishing. My brother was level 12 and he got over 20,000 gold from selling his fish.
At level 28, your options are rather limited. If you have beaten Krokotopia, farming Krokopatra's dungeon (the Tomb of Storms) should give you a good amount of cash.
If you do decide to follow my suggestions, I wish you best luck in your farming
On top of questing, add a small garden to your house or dorm room. Every harvest gives you gold and depending on the plant you may get some treasure cards that sell well.
