How to get rid of red marks/scars left from acne without going to a dermatologist?

Difficult...: Depends on the depth and color of the scars. For smaller scars you could try a vitamin E lotion. You may actually get more for your money with microderm abrasion or chemical peels.

Very difficult: The redness is usually due to underlying inflammation and increased blood flow to the areas. It takes 16-18 months for this to improve on its own. A non-invasive/non-ablative laser (laser genesis) works well in helping minimize the redness and also improving the texture of the skin and appearance of the scar. It requires anywhere from 2-6 treatments, and the improvement is apparent in 3-6 months.

Difficult : A good aesthetictian at a medical spa can do microbrasions and series of chemical peels. Remaining scars can be filled with a permanent filler.