Discount car speakers can be a good way to help build a great car audio system and save enough money that you can buy a more expensive head unit that has more power or features. However, there are certain things you need to look out for if you buy discount car speakers.
Make sure the speakers have a warranty on workmanship and materials if you're buying new speakers. If they're used, fully test them out to make sure they sound good and have the correct frequency range and can handle the proper power levels.
Obviously you don't want to buy anything that is damaged, especially if that damage is going to affect proper operation. If the speaker cone is creased, torn or separating from the cage at all, you need to look elsewhere. Cosmetic damage, such as chipped paint or a cracked grill is nothing to worry about, because they won't affect the sound quality.
Cheap car speakers probably won't perform as well as quality speakers. Have the sales person hook the exact pair you're looking at to an amplifier and stereo to make sure they work, sound decent and can handle the power level they are rated at. Discounted quality speakers are going to sound good, even in free air, and will handle the power levels they are rated at. Try a range of music with a range of frequencies and sound levels to make sure the discount car speakers can handle your proposed usage.
Weight/Size of Magnet
Be very wary of discount car speakers that say they can handle 250 watts but weigh less than a couple of pounds. Part of what determines how much power a speaker can handle is the size of the magnet on the bottom of it. Speakers with smaller magnets can't handle as much power without blowing out, compared to speakers with large five or six pound magnets.