Arcade unit for Gameboy Advance SP.
You place Gameboy Advance SP into the base of this unit, & it enables you to use arcade style stick & buttons with your Gameboy Advance SP.
Through some basic mechanics it transfers button presses on arcade unit to buttons on Gameboy Advance SP.
Of course this enables you to use it at a table, rather than having to hold the GBA SP in your hands.

Unfortunately the controls seem to get stuck sometimes, might wear in with use.
Due to this, the price is below wholesale cost.

Will not work with the older non-SP Gameboy Advance.

Also has these features (I'm not 100% sure of these features, as I don't have a GBA SP to test with) :
Speakers which connect to the GBA SP, with volume control
AA battery socket, which I think can be used to power the GBA SP (has plug inside to connect to GBA SP), with power switch