Trading on d2jsp in game such Path of Exile is against GGG ToS, so I guess you mean how you generate a link to send a "fake" trade in-game to the other trader so he can accept and this won't be suspicious.
2 options:
1. You create the link(like a normal pm you get from your trade) and modify it to fit to the trade with the user in d2jsp
2. The other user doing #1
for item
Quote
@<The char name you want to trade with >Hi, I would like to buy your <The item you want to trade from the user> listed for <# type of currency > in Ancestor
for currency
Quote
@<The char name you want to trade with > Hi, I'd like to buy your <# The currency you want to trade from the user> for my <# type of currency > in Ancestor