How do I get the most buck for my bang?

You should repair items before selling them, rather than selling many cheap weapons.

There are two reasons for this:

  1. You don't need to go back to town as often; weapons get heavy pretty quickly. You will definitely make more money-per-minute-played, considering the time to get to town, sell your stuff, and get back to whatever hole-in-the-ground you were exploring.
  2. You'll make more money by repairing items together*. This is especially true when the weapons are under 25% quality.

Once you hit a repair skill of 90 and level 14, it's worth picking up the Jury Rigging perk just so you can merge vaguely-similar weapons together (and convert the cheaper weapon into a higher quality on the more-expensive weapon).

*This depends on your skill, of course. I don't have numbers to back me up, but if your skill is below 25 or so, you may lose money. (But you'll still make more money-per-minute-played, because you're not trudging back to town as often.)