These will be mostly Ebay tips. About 80% of my wardrobe is from Ebay. I am not focused on designer stuff, but I do think there are good deals there on more high-end items. I don't know how their authentication measures compare to those of sites like TRR though.
Never buy anything without measurements listed.
Preferably measurements actually shown with a yardstick on the item in a photo.
Never buy anything without pics of the actual item (i.e., if only stock photos I don't buy).
I prefer Ebay to Poshmark -- at least in the app, Poshmark doesn't let you save very many searches. On Ebay I just set up a ton of very specific searches -- i.e., if you are shopping for pants, you can limit by size, inseam, rise measurement, etc., but you can include multiple sizes. So you can cast your net as widely or as narrowly as you want.
Yes to making liberal use of the "like" button/watch list -- on Ebay too, sellers will often send you an offer.
If you find something you like, hit the "show me similar items" button. Often you'll find the same item, sometimes even in the same size. I think this is useful because (1) you can identify which seller is offering the best deal, and (2) by comparing pics of the same item from different sellers, you might be able to develop a more accurate view of the actual color and cut of the item.
The real challenge is to develop the skill of seeing what is actually on offer rather than what you want to imagine is on offer.
Stay away from Ralph Lauren brands. (Self-interested advice since I am on a RL kick.) (OK I'm kidding, I hope you take that as the joke it is meant! Secondhand RL can actually be a screaming deal.)