I went in to buy some ammo for a bunch of weapons I got off a bunch of robbers and slavers, and went too far in the back of the arms dealer's place, and he started attacking after I paid for everything. How can I get him to stop attacking?

Try this:
* Make sure your character isn't in New Reno. (Go to Broken Hills)
* Download this: http://rapidshare.com/files/310166636/NEWR3.SAV.html
* Put NEWR3.SAV in ...Fallout2/data/SAVEGAME/SLOTXX (X= the slot number of your latest save)
* Please make a backup of your save before you modify it. (Better to be safe than risk lose your save)
* Start game and go to Eldridge.
* Tell me if this solves your problem.
Note: Unfortunately if you have a quest active with Renesco, or if you need him alive for any reason, you should not use NEWR3.SAV, because he is dead.

So which solution did you pick? Did the "leave the map and return" work?

Yes, running away was definitely a sound strategy. However, it didn't matter cause I unofficially patched the game, and lost my save data.