Okay, so actual shopping hints:
Use the search filters and keywords!
What are you looking for? Use those keywords in your search. For example, I've been looking for leather pumps in cognac. So I might type in, "Cognac Leather Heels" or "Toffee Leather Pumps".
If I get a lot of 'chaff' in the search, I might amend to "genuine leather cognac heels", etc.
Sizes: Poshmark, TRR, and ThredUp let you save your sizes. eBay doesn't save your sizes from one visit to the next, but it only takes a moment to select a category (say, women's skirts), and select your sizes there.
It also helps to know your current measurements. If you're unsure whether are skirt or jeans will fit, take the measurements on your current favorites that fit you well, and look at the fabric content on your item, and the item that you want to buy. Are both items stretch? Are both items non-stretch? It can help you figure out whether you'll get a good fit.
Also remember that if you're shopping vintage, the sizing was different prior to Y2K. The size 8 of the 1990s, is now the size 4 of today.
Sort: Decide how you want to sort your search. By price? By best match (on eBay)? By "Just in"? I've been shopping on eBay for nearly twenty years now (!!!), so eBay has me mapped pretty well, I think. So I'll often use "Best Match" to sort there, and set a price limit on what I want to spend.
Poshmark defaults to "just shared", which means sellers can re-share their items into the feed, which means you may see the same listing pop up again and again. So for Poshmark, I'll usually sort "just in" or "low to high".
Other filters that I frequently use are Color, Brand, Style, and depending on what I'm buying, the Condition (new, new with tags, new with defects, pre-owned, etc.)
Just a quick reminder again, that if you're on a set shopping budget, all of these sites allow you to set an upper price limit.
Anyway... I hope this helps!