1) I look for simple, and often looping seamlessly without a fade in/out
2) If the pack does not have a demo, Idk what I'm getting so I simply won't get it. If all/most of the assets have a demo and simple but descriptive names and are free, I"ll probably get it. A theme is also helpful, such as "intense pack" or "RPG pack"
3) They could cut out all of the white noise at the beginning and/or end. They could also make slightly different variations so it's easier to work with. They could also let the game dev know about a pitch range it sounds good in. Sometimes, game devs will randomly change the pitch of a sound so it's sounds more natural, have a range ready. Also, just offer to make VFX - they might assume you can only do music, even if you're not good at VFX, you are probably better than the dev so let them know.
Hope this helps. Feel free to ask any follow up questions.