This is what I did to get followers when I started out. It sounds tedious and takes time, but if you do it little by little as you are waiting online at starbucks or in an elevator or whatnot it will pay off over time.
In the app, go to "Shop". (This only works in the app, not on the main site that I can see). One of the options that will show up is "My brands." Pick a brand you sell, say Tory Burch. Then it is going to show you a list of people who are selling those brands as well. That means they like those brands and may be interested in your items. It will show you a follow button for each of those people. Follow every one! Many people, once they see that you are following them, will check your closet and follow you back. Some may not, but their items will show up in your feed when you log in. Share those items when you go into poshmark. Posh etiquette is to share back when someone shares your items, so then many of those people will be going into your closet and may like or buy your items, or at the least share some of your items so that they get seen by their followers.
Also, whenever you have mindless time such as watching TV etc share your items in general or to parties. Even if the exposure there doesn't help, the default setting when someone decides to go search for Tory Burch bags on their own is to show the items that were most recently shared first. So your items will appear at the top of the listings if they were shared most recently.