As I understand it, the Parthians had a large role in trade over the Silk Road as they sat directly in the middle of it. Two books I've read are The Persians and The Persians : an introduction and they cover the Parthian role (at leat a little bit) of taxing Roman goods passing through to China as well goods flowing the other direction.

I'm not sure how to phrase it, but there was an answer on here not too long ago that referenced. There is a book, "Through the Jade Gate to Rome: A Study of the Silk Routes", that has the quote I'm talking about. It says something to the effect that :

They trade with Anxi (Parthians) and Tianzhu (Northwest India) by sea. The profit margin is ten to one. ... The king of this country always wanted to send envoys to Han, but Anxi [Parthia], wishing to control the trade in multi-coloured Chinese silks, blocked the route to prevent (the Romans) getting through (to China).