Did some testing to look at the impact on buying & selling prices of increases to Charisma, Junktown Vendor Mags & Cap Collector Perk. Have a spreadsheet of data, but can't figure out how to attach.
Conclusions:
-Charisma decreases buying price (to player) and increases selling price by 5 to 6% per point of Charisma
-Junktown Vendor decreases buying price by about 3% per mag. No effect on selling prices
-Cap Collector 1 improves buying and selling by 10%
-Cap Collector 2 improves buying and selling by an additional 20% (i.e. 30% total)
-Selling maxes out at 80% of item value at a Charisma of 12 (with Cap Collector 2). Higher Charisma would be required to max out if don't have the perks, but max is still 80%.
-Buying minimizes out at 120% of item value