Help about trading

I have some questions about trading.
I never traded in this game and it looks a bit complicated to me.

1- How do I know how much do items I want to sell or buy cost aproximately?
2- Once I want to buy or sell something, do I have to create a prty with the person I to trade in game or is there any method from the page to trade? (Assuming I don't have the premium stash for trading purchased).

Once you trade, for ever will it dominate your destiny.

You will no longer be able to find items that will satisfy you. The items you trade for have been filtered out of millions of drops.

1. thats the hardest question in the game. takes time, experience, making mistakes, looking up prices at poe.trade. It's great and the true end game that has no ending.

2. @person right click and invite them. his hide or yours.

How do I know how much do items I want to sell or buy cost aproximately?

Really there isn't a "set" price on anything - it's an open market. That being said, poe.trade is the most commonly used trading platform out there, so it can be good to compare your items to the ones there. I imagine there are youtube guides out there on that kind of thing, but one way you can get a feel for it is to "price" some unique items before moving on to rares.

Over time you'll learn what's valuable on a rare item and what mod combinations to look for (life+resists, for example). The variance is so high I can't easily put it into words though.

Unique mods are static with different variable ranges (not to mention sockets and such can come into play), so they're a bit simpler to start with. For example, if you find a Marohi Erqi Karui Maul, you can search for those on poe.trade and sort by price. Take look at what other people are charging, and that should give you a rough estimate.

Once I want to buy or sell something, do I have to create a prty with the person I to trade in game or is there any method from the page to trade? (Assuming I don't have the premium stash for trading purchased)

The generally accepted practice is once you've arrived at an agreement, you invite the person to a party, then go to the seller's hideout.

Ok, that's nice. It's quite a scary thing to me since I don't want to be scammed selling or buying stuff.
I have some piece of equipment that I think I could sell at high price once I finished preparing it fully but I don't know If It's going to be better for me to use it or sell it.

OP also be aware of low drivers. What that is is people who list stuff super low and have no intention of selling to bait you into matching thier price. then they buy yours and pull all theirs off the market re-list at much higher price. It's very common now so pay attention when comparing prices.

Like if you see X item

at

1c
1c
1c
then all subsequent jump up to like 20c

the market price is probably closer to 20c

Another thing and its related. If you list something and get a quick bite you listed too low usually. Up to you whether to ignore that person and reconsider the items market position or stand by price.

It's a jungle out there. No rules. So keep wits.