I won't buy a loose disc ever if I'm looking for a game or come across it, expect in the exceptionally rare case of two reasons. One, I was given a really nice case with no game everything else there, or I got an expensive game ridiculously cheap (as disc alone or a questionably worn disc ) and I'll pay for the rest to still come out way ahead. I've done this a few times in the last year or two with the PS1. I was able to get the case+etc for free because the idiots at goodwill left the games out to be stolen. In both situations I paid around $15 for both Breath of Fire 3 and Diablo and I'm sure you're aware what those are worth.
Normally when it comes to game carts smaller than GBA (like Memory card sized stuff smaller) and optical I refuse to buy unless they're complete. For the cards I don't question if they'll work, but they're so small they are easily lost or damaged. And for discs, ignoring what I said before for exceptions, it shows me the former owner was stupid, a pig, or just didn't care and usually what I've seen are not clean discs, sometimes they look miserably bad in spots or all around meaning they likely won't read or skip out in play. Usually if someone cared enough not to thrash the easily cracked or scuffed jewel case odds are the disc is in pretty nice shape too which mostly holds true.