Motorcycles have only grown in popularity these last few years, and as budgets get tighter people wish to spend as little as possible on travel.  Thus, adding trailers to pull behind motorcycles will allow the rider(s) to skip paying for a hotel and bring their own heavy luggage.

The key things to look for while searching is size, price, and quality.  For size, make sure you undertsand exactly the dimensions that you're getting.  Don't just trust a picture that's shown, measure out the exact measurements for yourself to see if they fit for you.

For price, that of course is entirely up to the buyer.  However, if price is a concern, getting a used trailer for a motorcycle is definitely recommended.  This can greatly reduce the price that you would normally have to pay.

The final factor is quality.  Make sure the vendor is high quality and the product itself is of high quality (especially if you're buying used).  By following these extremely simple tips you'll be able to make your life far easier.  Good Luck!