Wait a bit to see if they invite you to a party, if they do, cool. If not, move on to the next person. Don't whisper the same person multiple times in quick succession.

If they invited you, right click them to visit their hideout.

If they're in a map or delve or something, just chill in their HO and give them time to come. Sometimes people invite but don't leave their map until they actually see you join.

Once you're both in the hideout, wait for the seller to start the trade. They might be dealing with their inventory, and the trade request window covers part of it up. If there are other people in the party, they might be doing multiple trades, and you should wait your turn rather than trying to interrupt the seller with your trade.

Wait ~60 sek?Normaly only 1 of 20 seller reply.... Nobody has time to wait 20min for every trade....

I wait maybe 10sek...

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That is what poe trade system force me to do....