Should take 30 minutes for players who can't teleport to Varrock adn Falador since all can teleport to Lumbridge.
All you need is 14k or 14,000 coins to get your 30k. You make a profit of 17k (17,000 coins) in 30 minutes or less.
Now what will you be buying for 14k?
The answer is easy. just go in to the most empty world you can find and teleport yourself to Lumbridge if you are in anyplace other then or near Falador. If your in Falador then stay there.
Step 1: Lumbridge
Buy all of the feathers, but buy them 500 at a time. So only two times should do.
You can bank them if you want, but it isn't necessary.