Yes, it's a scam. You are to never text/talk with someone "off-site".
All communication must be done through Poshmark.
Never give out your phone number/email.
They are doing a phishing scam, trying to steal your account/identity/bank information, etc.
From Google:
Some Poshmark scammers may comment on a seller's listing, pretending to be interested in the product. They may claim that the pictures posted “aren’t displaying well” on the website or app and ask the seller to email them clearer photos. However, once you send the email, the scammer will send you a phishing email to try to steal your personal information.
A few ways to spot a Poshmark buyer email scam include:
A buyer insists photos are blurry when they look fine on your device.
Comments from a buyer appear within minutes of you listing an item.
Their profile shows a low follower count.
The buyer asks you to contact them via the email in their bio.
The buyer asks you to text them