whats the deal on buying used, used knives

Yes. I said used twice on purpose. I'm curious what others think about buying used knives from places like craigslist and ebay from people you dont know. Sure, you can see if the knife is cracked, and if it is I could repair it - but some things you cant see - like ... I guess what I would call "heat damage". Where the knife has been heated beyond a point that would mess with the temper and the toughness of the steel.

In addition, what if you buy a used knife - fix it up, rehandle, and someone wants to buy it? Would you be kosher with that?

I ask because I come across people wanting to sell me knives they cant use, or find knives on craigslist for pretty good rates. I saw a decent Mac for 50 USD.

If you want to take on other peoples cast off's to rehabilitate/refurbish then you are more of a "project guy" than I am. I'm sure you may come across some gems this way but would think most would be ahhhhhh, crap.

Not sure of the intent of the "Would you be kosher with that?" question.

If you're asking about reselling refurbished knives in the Ebay, CL markets, I'd say the more power to you, sounds like a fun way to recoup some of the hobby expenses.

If you're asking about refurbishing knives and selling them here through B/S/T, that would be considered an abuse of the forum. B/S/T is reserved for members to sell their personal knives (and stuff), usually to raise funds to buy more knives (and stuff). Of course there are avenues such as becoming a hobbyist or craftsman here that would change all that.

I'm not ascribing any motives and want to thank you for the opportunity to publicly discuss this.

Have fun.

Not sure how to look for heat damage...but maybe an example of an overheated edge:
I bought a few vintage American knives off ebay auction who obviously had access to some guy with a belt sander. All the patina/rust/pitting of vintage knives yet a shiny convexed looking edge. I am now suspicious of any time I see that on an auction that it's going to be overheated or otherwise messed up steel at the edge. Anticipate having to lose a 1mm or much more trying to sharpen and form a stable edge and not have the metal just feel crunchy or otherwise just dissolve on you. Also expect issues with knife profile from poor maintenance or steeling, broken tips, reverse bellies to reprofile, and probably a good deal of thinning.

And it should go without saying if it looks like there's no edge or a ragged looking one...that'll take some abrading off as well

I bought a lot of old carbon knives when I was learning to sharpen on stones, bought even more crappy knives (stainless even) when I was learning to use a belt grinder and now leaning to rehandle. Cheap is good when you are learning!

There's always the thought of "where might it have been!", especially with knives that have non-watertight handles and/or non-polishable finishes and in styles that are typically used to work on potentially unsanitary ingredients (fish and chicken knives especially)...

Strange thing about our local ebay (german) is how little there is in valuable ex-drawer queens... and how much off-brand supermarket or knockoff used cutlery is floating around there....

If you're doing it for fun and experience and are willing to absorb the costs, then:
http://www.kitchenknifeforums.com/showthread.php/13407-PIF-Volunteers-Needed