In reselling, I believe you're either a serious reseller or you're a hobbyist. The funny part is, everyone thinks they get to decide which one out of the two they are. The problem with that is EVERYONE can claim they're a serious reseller, and they would be right if we're going off of that standard. Hopefully, you caught the major flaw with this way of thinking.
The Market Got You Covered
You don't need to call yourself a serious reseller or a hobbyist, the market will take care of that for you. No matter how much you call yourself self-employed, boss, Poshboss or any other related moniker, the market doesn't care. Again, the market will let you know who you are. This means if your work ethic is mediocre, the market will give you mediocre sales. If you only list when you can, the market will only give you sales when it can. YOU CAN'T DICTATE WHAT THE MARKET WILL LABEL YOU AS. The most important part is, the market is never wrong, ever.
Quit Making it Complicated
It's quite simple, work your ass off on Poshmark or any other reselling platform for that matter, and the market will reward you. Make sure you buy the right items at the right price and consistently list with good listing practices. If you're looking for the silver bullet, that was it. I'll keep this short to prove my point of how easy it is to win over the market.
Your words won't win this argument of whether you're a serious reseller or a hobbyist, your actions will.