Should I sell my Dslr and get a different one?

I currently own a Canon T3i with 18-55mm & 50mm f.1.8. I've been looking at a Nikon D90 and I love the image quality and things like that. So should I sell my Canon T3i with the 2 Lenses and use that money to buy myself a used Nikon D90 on craigslist with 18-55mm lens? Or should I stick with my Canon T3i?

Just the cost of switching camera systems requires there be a compelling reason.

If you are confused about what you will be gaining with such a switch, you need to spend a lot of time visiting websites like DP Review and dxomark.com/.

You also need to spend some time on both the Canon and Nikon websites comparing all the lenses each camera brand makes that you think you will need down stream

Trading a camera model that's only a year and a half old for one that is MUCH older will not improve your photos. If you can't get good photos from your t3i, you won't be able to get good photos from a D90 either. Learn how to properly use a DSLR instead.

Repeat this 100 times: "It isn't the camera and lens its the photographer." Canon T3i or Nikon D90 you'll get the same results. Canon 18-55mm or Nikon 18-55mm you'll get the same results. A different camera and lens will make no difference.

What will make a difference in your photography is having knowledge of these four elements of photography: Light, Composition, Exposure and Depth of Field and the skill needed to use that knowledge so you can make the pictures you want to make.

Light
http://photographyknowhow.com/quality-of-light-in-photography/
http://photographyknowhow.com/direction-of-light-in-photography/

Composition
http://www.photographymad.com/pages/view/10-top-photography-composition-rules

Exposure
http://www.digital-photography-school.com/learning-exposure-in-digital-photography

Depth of Field
http://www.cambridgeincolour.com/...-field.htm

Good photography requires thinking. You have to train yourself to think about Light, Composition, Exposure and Depth of Field BEFORE you release the shutter. All too often, digital cameras tend to encourage people to just point & shoot point & shoot point & shoot point & shoot. If this is a habit you've developed then its time you broke it and began developing the habit of thinking about the previously listed four components of photography.

"It isn't the camera and lens its the photographer."

Changing camera brands will not improve your photography.

There's virtually no difference in image quality between cameras of equivalent level from the various brands. Nikon, Canon, Sony… They all make excellent cameras. You can create amazing images with your T3i if you learn the art and science of photography.

I shot these with a Rebel T1i

These were shot with a Rebel XSi and the kit 18-55 lens:

It's about understanding exposure, composition, light… Not about how new your camera is. Sure, a newer camera model or a higher end camera from the same brand will give you better options, greater flexibility in terms of dealing with extreme shooting conditions but cameras don't take great images… Photographers take great images.

Also, switching from a brand new T3i to a used D90 doesn't make sense. You are loosing money by selling your canon gear (you won't get what you paid for it) and you are buying something that has been used and potentially has hidden flaws (burn pixel on the sensor, fungus, sensor overheat damage, water damage… ) that is out of warranty.

You can be sure that I would NEVER bu a used DSLR off of Craigslist. Used at a REPUTABLE camera shop yes but not Craigslist.