How can trading phisical goods in the blockchain be safe?
Simple answer is that both parties need to trust the escrow! Their is no smart cryptographic technique you can use to ensure a link between maths and physical goods in the real world!

If you're talking about digital goods, or products with DRM, the issue is different, but when you're shipping a physical product, like a phone, without DRM, then you need good old fashioned trust. Now if you use a 'reputable' escrow agent on a 'reputable' site, then you can reasonably trust them.

Customer reviews also play into this. This is how sites like AlphaBay could exist. You have no idea that your illegal product is going to arrive in the post, after all, you're dealing with a criminal by definition! However, the interests of the buyer, vendors and site align to ensure that everything works most of the time... See:
Jamie Bartlett How the mysterious dark net is going mainstream