If you are looking to sell/trade-in your Game Boy Advance SP you first need to figure out which type of model you have.
The only way to check exactly which model you have is to look at the screen.
The original Game Boy Advance SP has a front-lit screen and is the dimmer model called the AGS-001. This frontlight tends to have a dim blue light that can be turned on or off completely.
The later AGS-101 model is the brighter backlit version of the SP. This screen will be very brightly lit and similar to screens of newer models like the DS lite. They also have a dimming button but the light will not turn off completely.