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.

What kind of things are you specifically looking for in your game music? 

Music that loops well and adds to the game experience without distracting players

What are things that may make want to or not want to purchase the music?

If the music is distracting to the player, for example, if it suddenly gets louder or changes tempo drastically. If I want the music to change drastically, usually it would be a scripted event that I'd want control over.

What are some things a games composer could do in their music packs to enhance your experience when developing games?

Its nice to have similar motifs in the menu, peaceful, and boss music. It helps that game feel more cohesive.