I don't buy collector's items because I like using the things I own, but I think this can be sold for more than $300. Heck, there are still places selling used GB Micros for $100, this could easily fetch $500-$1000.
HOWEVER, seeing as the GB Micro used an internal battery, this unit might be in a non-working order if the battery has fully discharged, and it's a shame we can't know if it's still working without opening the box. Still valuable as a collector's item, though.
Good luck with your sale, dude (or should I say, THE Dude). This is a steal at that price, and I hope console collectors will get a bidding war started on it.