How do you check the popularity of items if possible at all

Looking around the Market and Items forum, try to see what people are selling & buying the most. There is no official function to check for that afaik, dont even know if such a thing would be possible

You can't, actually. You'll have to use manual labor (of yourself, ofc) and look at the first 5 or so pages, though there are some items that are always wanted, such as forces and relaxes like hunter.

Going to any item in the shop will give you stock, qi, price, owned by, total items, and total in circulation.

The total in circulation is the number of times this specific item has been sold in the last week (maybe month but I'm pretty sure week). So when an item has a higher circulation, it get's traded around more often so it's more populair.

Picking a random item from the shop here: http://forum.toribash.com/tori_shop_item.php?id=828 you can see that adamantium blood has a circulation of 14, which isn't very much as it's not the most populair item.
Compare it to a more used and populair item: http://forum.toribash.com/tori_shop_item.php?id=88 the gladiator force has a circulation of 648, so it will probably also get sold pretty easy and is kinda populair.

*Total in circulation tab shows how much times specified items were transferred between accounts within last two months. Not a week.

This number includes all the transfers that touch this item - from transfers between some player and his alt, to transfers while purchasing the item from somebody / from the shop.

If the number is 60, it means that items of this kind were transferred ~one per day.

This is not really true in most cases. It just means these items aren't being kept or are being lost in duels (quite a lot of items that are being kept are items that people want, but there aren't many available), and are being traded to someone else. It doesn't really specify how many times a day, etc they're being traded either ... so it doesn't really help show which items are popular in any sense really.

The only way to check is to see what items people are looking to buy, or what they like to buy. However in my personal opinion, selling on the forums is overrated if you're trying to market because you can always get more TC from selling it on the actual market. It just may take a little bit longer to sell it there, but if you have the TC you can afford to wait to sell it. You just have to make sure you're bumping those items you own every now and then and making sure your price is the lowest. It's not complicated at all really. If you're looking to profit just buy items cheap from people that are needing the TC as they tend to give you a cheaper price (usually ends up being a dueler or someone who is a bit off of getting an item they've been trying to get for a while) and then selling it on the market.

If you see someone buying a specific item or set of items that you own is when you'd want to try and sell it on the forum, if of course you can get the person to pay enough for you to profit from.

It's basic marketing, just figure it out.

Yeah, this question is basically answered.
Closing.