She probably stalked the other seller's listing and found you commented on the item you purchased. So technically, yes they can see what you buy as long as you commented something on the purchased listing.

The only way she should be able to see your purchase is if you relisted it for sale on your closet. Or if you brought it from her. Every time you sell something even though it sells that listing stays in your closet but has sold written on it you cannot remove the post I tried and it wouldn't let me. But they shouldn't be able to see your profile or your personal purchases.

She could have also seen a review/rating you left for the seller!

Once an item is purchased, it says "Sold". Other people do read the comments under the "Sold" items to get a feel as to how the seller operates. But, if someone purchased the item right after You made a comment it doesn't necessarily mean that it was You who purchased it. There is no way anyone knows what You buy on there technically. If a seller posher sees You commented on someones item... they figured You were the one who bought it, so if they have something that matches it, they message You. Always ask them what are they talking about? What item? Who was the seller I bought something else from?
If they are a team (sisters with sisters selling... friends with friends selling, etc...) then they would actually know what You bought. Alot of scammers as well on Poshmark. Search up top the word "Scammers" "Scammer Alerts"... Your mouth will drop!

No one can see what you buy and you cannot see what other people buy unless it is commented on. For instance you can purchase something without "liking" it or commenting on it and no one will ever know you bought it except the seller. If you chose to give them a "love note" that will be shown to the public on their "about me".