Another way...let's use an example that you are looking at some Malabrigo yarn...and you like the colorway that "therewillbesnacks" has to offer for sale or trade:
....so you've found the yarn in yarn searches...click the owner's Ravelry name at the top left of the yarn "3x5 card"...click here to view what I mean....then after you click the owner name...it will take you to the owner's Malabrigo yarn page...click here to view what I mean....then click the SEND MESSAGE envelope to ask "therewillbesnacks" about her yarn.
When she gets your message, the yarn you are asking about will be linked to the message.
People who are selling a lot of yarn need to know what yarn you're asking about. It's happened to me where someone messaged me about some Elann yarn...and I had no clue which one she was asking about. It took a further two messages back and forth, several days, for us to get it sorted out.