For the record, EarthBound isn't rare, just overpriced. It sold 140k copies in North America (Wikipedia), so while it didn't sell very well compared to other Nintendo titles, it is nowhere near the rarity of say Tooth Protectors (Atari 2600), Super Noah's Ark 3D (unlicensed Super Nintendo), Tetris (Tengen, unlicensed NES), anything on Sega Saturn, all of which can be found on eBay for comparable value, if not cheaper.
Also it is unreasonable for mainline Pokemon games to have outrageous price tags given that every one of them have sold millions of copies. Yes, I am aware of "supply and demand", but it doesn't help that a lot of these game prices are artificially inflated when there are plenty of these game copies to go around, and the "demand" only really exists among collectors as anyone who just wants to play the game has probably already done so through "illegitimate means".