It comes down to intent
... and possibly responses to illegal activity.
Localbitcoins intention is to facilitate trade in bitcoins - in itself a legal undertaking. Cryptocurrancy transactions can be used for illegal purposes but it is not the sites intention to enable that.
Amazon.com intention is to facilitate trade in books - in itself a legal undertaking. Some books contain material that is prohibited under some countries laws but it is not the sites intention to enable trade in illegal books.
Ebay.com intention is to facilitate trade in "stuff" - some "stuff" is illegal but it is not the sites intention to enable trade in illegal stuff.
Silk Road intention was to facilitate trade in illegal goods and services - therefore they are accessories to the crimes committed.