Can I get PCs on these items, please?

The jellyfish aquarium comes from the July 2015 Collection, so it's worth a bit less than what the collection is going for typically.

The plushie is from the July 2012 Collection, same deal for pricing.

- I'm not sure how to know the prices. I have barely played here much and don't quite know how I can into contact with the items.

http://www.subetalodge.org/ is very helpful for finding out where items come from. Because you can purchase a July 2015 Collection (https://subeta.net/ushop.php?act=dosearch&itemname=July+2015+Collection&type=shops)for about 25m in shops and then choose that you'd like to receive the Jellyfish Aquarium from it, then the Jellyfish Aquarium is worth as much as the collection or less. Usually people will pay less because the collection has the potential to be numerous items, but once you choose the Jellyfish Aquarium(or whichever other item from it) then you're stuck with just that item.

You can look them up in the Shop Search, also if you do look items up on Subetalodge like said, they have a direct link to the search for the item.