I assume as we are in the GBA section you are talking about the 3 in 1 GBA (the full name of which is the EZ5 3 in 1 expansion pack) slot device rather than the DS slot device known as the EZ5.
They were never worth a great deal and if you just dug it out then chances are the battery will have to be replaced. If the battery is good they are not undesirable and you could probably get £10-15 (shipped) if it is a full size GBA one, the DS lite sized one is a bit more common though no less desirable so you may face competition and reduced prices. Perhaps a bit more if you sold it in real life.
Given half the people I have bought and sold things from seem to live in magic fairy land I am not discounting the possibility.
I was thinking more that such things are well known to those of us that inhabit the online world. Selling it to a kid in school (most kids seem to have a nasty R4 clone) or a bloke down the pub (for said same kids) tends to be a touch more lucrative and easily done.
Hold up a minute. You might have always just used it in PSRAM mode and not written anything to the NOR and so the DS will not have anything to detect. Likewise even if the battery is dead then someone may still offer a token sum plus shipping at least.
ok, Battery is still good just tried it with GBAExploader and gatway blue card, flashed to nor turn off and removed gateway and turned back on and gba game loaded fine, as FAST6191 said was just empty so nothing detected