Most stores that have electronics have several cameras that range from $90 and up. I've never owned a camera, much less a digital one. Anyway, I was thinking of buying one to take some pics of some guns I may sell online, and to have when I'm out and about. Walmart has a Canon for $90, and I've heard that Canon is better than Sony, Nikon, etc. What do I look for if I want a camera that takes good close-ups, like the marking on a gun?

Everyone has different requirements.  Check out Steve's Digicams.  They have a ton of information on each digital camera, so you can pick the one that is best for you.

Most low end cameras are adequate at best for your purposes.  Their primary function is to take low res photos for facebook or similar.  Quality is rather poor, but it might be adequate for your purposes.  People who want 'good photos' abandoned them long ago.

This being said, look for cameras with a decent macro setting, this will allow you to focus during extreme close ups.  Only trust optical zooms, never digital.

All point-and-shoots are pretty much the same, just get a Canon with good reviews. More important in taking your gun pictures for sale will be good lighting, steady hand (or a small tripod like one of them gorillapods) and knowing how to use the macro mode.

Like you i wanted one to post pics of crap on line. My last camera an HP just crapped out mid pic taking.
after humping around on line and 3 different stores. I hate to say it but the $125.00 and cheaper are give or take within $10-20 between the 3 stores i listed. I went with the Kodak from wally world as an extra years warranty, if the camera takes a shit was only $6.00 vs $20-30 at the other 2 for the same coverage. YMMV.
worst buy
ultimate electronics
wal mart

do you want to use AA batteries or the Li-Ion rechargeable where you plug in the camera to recharge internal battery, like a cell phone
i just purchased a Kodakwhatever 14 mega pixels 3.5 optical lens. @ 6MP's i am very happy with the quality. not a super macro photo taker, but it gets the point across.
the 2 pics are of an SP101 in 22lr. best digital pics i've taken for under $100.00. Not quality wise like my old K100 SLR, but got to love the feeling of instant gratification

That Canon that Walmart has looks ok, Canon PowerShot A495 Silver 10MP Digital Camera, 3.3x Optical Zoom, 2.5" LCD Display.

Seems like the macro setting is good, that is what you'll need for gun photos. Flash will be adequate for most things, but don't expect too much.