How do I know when buyers are interested in my item and how can I follow up with them?

[coolections]

You don't. Ebay does not allow you to contact watchers. It is up to them if they want you item.

[kensgiftshop]

@sdcoinsandstamps 

You'll know when someone is interested when they buy an item or place a bid.

[mtgraves7984]

They'll buy it and you'll ship it.

[m60driver]

Here is your Ebay 101 lesson for the day.  Go to the the selling page and look for the link to the eligible offers page where items with watchers are listed, and even items that were just viewed, and send an offer.  I find it is one of the most effective methods for generating a sale.  With the offer you can send a message.