well, usually the person that have been less on forum is going first on these kind of transactions, becouse noone knows you. People who trade a lot and have a lot of posts and got account created long time ago they are 99 % trusted people, they don't want to risk their reputation to scam for one trade, really. Sometimes they will go first without even asking, but sometimes u will be asked to go first just becouse u got new account there and not too much activity on d2jsp recently. But honestly, i have been traded over 500 times here and on PC PoE, even on CS:go keys and NEVER been scammed, so its very rare there. So don't worry
About d2jsp and trading in general - forum itself its not supported by GGG, but we live in harmony - don't mention about it in game, don't spread a word about RMTing or anything like that and nobody will ever ban you for these kind of trades, especially if u don't do it on big scale.
RMT sites usually got similar prices as there, sometimes u got better ratio there, sometimes on sites. But honestly, if i could choose, i would choose getting my stuff there - more safe, more easy, and you trade with real people, not with chineese farm bots.