If you both have good feedback then go with the cross-ship (both ship at the same time). If one party has no feedback, it wouldn't be unusual for the no-feedback party to ship first. If there is a price discrepancy (e.g. trading a $300 item for a $200 item, so one party pays $100 via PayPal) then it is usual to pay the PP up front, although if they have no feedback at all, it might be agreeable to only send a partial payment up front. There is always a degree of trust given to the person who posts the thread vs. the person who replies, especially if they have photos of the things they are selling. Also PayPal offers some recourse, so it is easier to trust somebody when it is a straight purchase as opposed to a trade.
I have done a trade/purchase once before with somebody with no feedback. It was my thread: I had my 50mm f/1.8 for sale and indicated that I wanted to trade + cash for a 50mm f/1.4 if possible. At that point, I had 2 or 3 good feedback where I had sold things in the same price range (e.g. Sigma 30mm f/1.4). So I was buying/trading his 50mm f/1.4 for cash + my 50mm f/1.8. I sent half the PP, he sent the 50/1.4, and when I got it, I sent the 50/1.8 and the rest of the PP.
Basically it all comes down to trust. I have had no problems with the people on this forum, either buying or selling (and I have done about $1000 each way). Look to their feedback and if they have a couple of good ones as a seller then that's good enough for me. Also of course it is harder to trust somebody when they are selling a really big-ticket item like a $3000 lens or something. Fortunately there are a lot of pro/semi-pro photographers on this forum, often with their own established studios, websites, etc. and they have made a lot of posts so it is easy to see that they are who they say they are, and that they have what they say they have. Trusting others is a personal issue and it is up to you to decide where to draw the line. This forum has little in the way of recourse, so if that scares you then you might want to stick to eBay and local sales via Craigslist where you have some measure of control.
EDIT: My advice has a bit of experience behind it, having been active on several other forums. There is a website called HeatWare
which is a community of forum trader feedback that can apply to any forum around. Unfortunately it does not seem to have taken hold here, but it is pretty common on tech sites where people sell computer parts, etc.