What are examples of sought after/out of print/rare/whatever manga volumes?
It can be hard to pinpoint, as series popularity can wax & wane, and titles do get republished. the baselines for finding rare stuff:
-last volumes cost more than first
-Shojo costs more than Shonen
-The newest version, if the title's been rereleased, is more expensive.
I've seen the last two volumes of Sugar Sugar Rune regularly hit the $80 mark, that's the priciest I personally have come across. Random volumes of Bastard! can run $30-50 (that whole series is out of print, so prices flux a lot). Sometimes long-running series like Hunter X Hunter can see random volumes spike in price if stock runs out... these don't last since a reprint is usually not far off.
I will say this much- don't buy anime or manga speculatively. The market is
volatile. Popularity rotates quickly, the abundance of free/cheap streams and downloads (legal or otherwise) makes it hard to see what's popular and what's profitable, and titles can go in and out of print too fast for one to capitalize on. I remember years ago Rightstuf did a bulk blowout where you could get 25 DVDs from a specific list for $100. One of the series in it was Princess Tutu, but only volumes 2-6. Worked great for me, I got volume 1 for Christmas. People threw it in as order filler, watched what they had, and discovered a surprisingly dark fairy tale under that cutesy name. End result? Prices for volume 1 spiked to well over $100 as people clamored to finish their set. Within a
week of the price spike, the company announced a complete set re-release coming in a month. Prices stayed high for a little bit (no idea if any sold though), then it all crashed down after the re-release. I was paying attention for a friend interested in the series... if that hadn't been the case, Id've missed the whole thing. At this point, the popularity window is long passed and you can get the whole series under $20 easily enough- which is less per disc than the big blowout sale that started this lil' rollercoaster.