The shop interface could use a bit of tweaking. If you stand in front of the item you want to buy and hit space you just dash. You have to rub yourself against the item and then hit space. This will then dash *and* buy the item.
Yea the store was an older feature before some game design changes I made I'm fixing this now in update v 1.0.3 should be up really soon. I'm also adding more stores around the world and dungeons with variations.