I wrote a post several months ago about online consignment. After months of experimentation, the verdict is in -- it's definitely not worth your time. (Specifically, it's not worth your time as a source of regular income. If you have clothes laying around already, go ahead and give it a shot.)
The biggest issue that I've noticed is how specific the market is. If you don't have the capital to invest in high-end designer clothing and accessories, you don't seem to make much. Over the course of months, I only managed to sell some comfy worn-out clothes to a few grandmothers for $1-2 per item, and one dress for about a third of retail (after Poshmark took their cut, it was more like a fourth).
Another issue is the fact that people just don't expect to pay a fair price to an individual. Unless your entire inventory fell off the back of a truck, you're likely going to take a loss.
At the end of several months of trialing different platforms, I spent over $60 on inventory and made less than $40 back. Of that $40, most of what I sold was clothing that I already had -- the majority of my inventory has been given away or packed in storage at this point.
If you have the capital and access to get designer clothing at deep discounts, you may have better success than I did. I definitely saw some items sell for pretty large amounts, but the capital needed to acquire that kind of inventory is more than a little intimidating.
All in all, it seems that you'd be better off looking for a different avenue of income if you were looking for a lucrative side hustle.