Generally, I look first for desirability. There is a ton of military gear that may be decent quality, however, it is redundant or absolutely useless for most people (Looking at you cheap fabric M9 drop-leg holsters). Next, I look for recognizability. People need to know WHAT the item is, who made it, and possibly a little history on the item. I often purchase stuff that is a gamble as there is not a lot of information out there on it, but something sticks out to me about it. Maybe it’s the manufacturer, maybe it’s a strange or uncommon camo pattern, or maybe it is the possible usages that I can see from my background as an outdoorsman. A perfect example of this are these Mystery Ranch Load Sling packs located here https://otsurplus.com/products/mystery-ranch-load-sling-very-good-condition?utm_source=copyToPasteBoard&utm_medium=product-links&utm_content=web
I know that many outdoorsmen, myself included, do not have a lot of time or money to put towards trying out lots of different gear to find out what is durable or works for us. Because of that, they often stick with military surplus gear because it is generally a known quantity. You know what you are getting, you can find all sorts of reviews and video detailing usage, modification, and repairs. There is also the group of veterans, usually combat arms, that believes that they should keep a set of gear handy. These vets typically want a set of issued gear that they are familiar with and that they have bet their life on before.
The final caveat is price. We must purchase at a price that is low enough to cover freight, marketing, storage, listing, and all of the other costs associated with retail and still turn a profit. In this current market, with money getting tighter for most people, and many sellers buying simply to send to current conflict areas, acquisitions are getting more and more expensive.