Yeah, I don't touch boxed games because...let's face it...who kept em?
I know I didn't.
Because I couldn't fit the boxes in my travel case
As far as obscure titles. I haven't noticed many that obscure. Maybe Wisdom Trees and Sachen carts (which I have never seen). See the thing was with Game Boy. Everyone had the same core of 5-10 games, but everyone always also had 5-10 other random titles. I know at least 5 friends from school who had Gameboys, and except for a core of 5 games, we didn't have a single duplicate cart in our collections. So I see a wide distribution of all kinds of titles in my travels.
As far as the rarity list on DP. I tend to think it's a good indication of rarity.
Although in my area, I tend to see a flatter rarity grade...R3 and R4 carts are just as likely to be seen as an R1. A few of the R5 and R6 games strangely enough may as well be 8s or 9s. I just never see those.
One thing I do want to note is that I really don't see a correlation between rarity and prices like you do with other systems. Except for maybe the Wisdom Tree, Tetris, and Sachen (if you ever see these) carts everything around me gets pretty flat pricing. An R1? $5, an R5? $5. etc. etc. Tetris? $15. Every time.