Fallout – How to get the NPC ID

Here are some merchants you can sell stuff to:

Abernathy Farm - Connie
Drumline Diner - Trudy
Diamond City - Major hub/Many vendors
Good Neighbor - Minor hub/a few vendors
Grey Garden - Supervisor Green
Bunker Hill - Minor hub/a few vendors

Also vendors will visit your settlements (such as sanctuary) on occasion.

You will get a marker for Diamond City relatively early on during the quest:

When Freedom Calls

Obviously there are ways to increase your income, but I'm considering that outside the scope of this answer, and focusing on getting better prices.

Discounts

Some vendors will give you discounts for actions taken in the game, like Carla giving you a discount for making a sarcastic remark when you meet her. Shopping at vendors that give you a discount will help.

Raising Charisma

Much like raising intelligence for additional xp, you can raise your charisma for better prices. I keep a set of charisma gear on me, including clothes (+1-3 CHA), a hat (+1 CHA), and glasses (+1 CHA). I put these on before speaking with vendors, and it improves both buying and selling prices.

As an experiment, my character with 3 CHA sold some items without the CHA equipment (3 CHA), then I loaded and sold the same items with the equipment (7 CHA). I got approximately 200 caps without, and approximately 250 caps with.

You could also drink some alcohol first, which should pay for itself.

Perks

There's a perk for better buying and selling prices, called Cap Collector. I haven't tried this, but the exact numbers are listed on the fallout wikia.

You can also improve buying and selling prices with the perks you receive from finding Tales of Junktown Jerky Vendor magazines.

Other Items

Some specific items will give you a boost specifically to prices. Grape Mentats will give you 10% better prices and +5 Charisma for 8 minutes.