I'll second Rini's point that in Japan "older series" amounts to a
very short time ago. My standard used to be that by the time something had been licenced and hit the shelves over here, it was long gone in Japan. That may be different now, but you get the idea.
what about series like working! or hakuouki? stuff like that?
I'm not the best one to look to for "taste in popular trends", but here's some ideas. Note that there may be some top-selling series out there which are underneath my radar since they haven't tweaked my interest (and stuck in my memory)
"Hakuouki" will probably be a hit with the bishi-boy loving crowd, plus those into the late Tokugawa era.
"Working!" is an amusing comedy set in a family restaurant, probably not as strong a "goods" draw, but what do I know.
Others would be:
"K-On!" (and the second season "K-On!!") is showing a lot of popularity, and I know there are folks who will want goods on the girls in the band.
"Angel Beats!" had some popularity, and I can see similar appeal, but some folks may be turned off by the ending.
"So-Ra-No-Wo-To" (Sound of the Sky) may turn out to be my favorite this year, but that probably means the average customer won't like it as much.
"Macross Frontier" had their first move last November, and the second should be coming out this year, so you might be able to find some goods for that. And I know it's popular.
And to jump to the other end of the scale, here are some shows beginning this season (like this week).
"Amagami SS" is adapted from a romance simulation, you'll have a cast of cute school girls to customise goods.
"Ookami-san ot Shichinin no Nakama-tachi" (The Wolf and her Seven Companions) Looks to be a good comedy with a cross between Fractured Fairy-tales and odd school life.
"Sekimatsu Occult Academy" a cool beauty had to deal with a time traveller and the occult
"Asobi ni Iku yo!" (Let's go play) Cat-eared, curvacious alien girls fall into a guy's life: comedy ensues.
Several more dealing with horror/demons/zombies: "Noraihyon no Mago", "High School of the Dead", "Shiki": not my genre, but others are looking forward to these
And for the Fantasy-RPG crowd, "Shukufuku no Campanella" and "Densetsu no Yuusha no Densetsu" (Legend of the Legendary Heros).
There's also a couple of sequel-series which I'd recommend stocking up on if the first season sold for you. Yes, Hollywood doesn't have a lock on sequelitis, but it may give another round of goods. And in this case, you have marketing data from the earlier series. The ones that come to mind are: "Black Lagoon: Roberta's Blood Trail", "Kuroshitsuji 2", "Sekirei - Pure Engagement", "Strike Witches 2", and "Sengoku Basara 2" [same target audience as Hakuouki]
I fear I may have given you too much info, rather than too little, but there you go. If you've got more questions, drop me a PM.