Trading is the art of negotiation.  When you conduct Diplomacy with a CP - you have the option to buy and sell goods based upon what is available.  Each time you enter a CP to Conduct Diplomacy: you will receive the current status of your Trade-Status (different from Attitude) with the CP in question.  All On-Map CPs buy and sell at Standard Status (unless a World Update dictates otherwise.)  Off-Map CPs nearly always begin at Reviled status.  As you spend more Gold or sell more Gold-worth of goods to them: this status will improve.
(Alternate View for those playing on Desktops: Trading1 )
Conducting quests with a CP can improve this as well.

The following are the trade values for resources.  For the trade values of other items, see: Trading Items.
For Buying FROM a CP
Stone and Lumber
GoldStoneRatioTrade Status
111.0Reviled (Rvd)
231.5Poor (Por)
122.0Subpar (Sub)
133.0Standard (Std)
144.0Preferred (Pfd)
294.5Favored (Fvd)
155.0Best (Bst)

Iron
GoldIronRatioTrade Status
430.75Reviled (Rvd)
111.0Poor (Por)
451.25Subpar (Sub)
231.5Standard (Std)
122.0Preferred (Pfd)
252.5Favored (Fvd)
133.0Best (Bst)

Foodstuffs
GoldFoodRatioTrade Status
510.2Reviled (Rvd)
410.25Poor (Por)
310.3Subpar (Sub)
210.5Standard (Std)
320.67Preferred (Pfd)
111.0Favored (Fvd)
231.5Best (Bst)

For Selling TO a CP
Stone and Lumber
GoldStoneRatioTrade Status
155.0Reviled (Rvd)
294.5Poor (Por)
144.0Subpar (Sub)
133.0Standard (Std)
122.0Preferred (Pfd)
231.5Favored (Fvd)
111.0Best (Bst)

Iron
GoldIronRatioTrade Status
133.0Reviled (Rvd)
252.5Poor (Por)
122.0Subpar (Sub)
231.5Standard (Std)
451.25Preferred (Pfd)
111.0Favored (Fvd)
430.75Best (Bst)

Foodstuffs
GoldFoodRatioTrade Status
144.0Reviled (Rvd)
133.0Poor (Por)
122.0Subpar (Sub)
111.0Standard (Std)
320.5Preferred (Pfd)
210.3Favored (Fvd)
310.25Best (Bst)

HQ Resources (Lumber, Stone, and Iron) are treated as regular Resources when sold to CPs.
HQ Foodstuffs are treated as 2 Regular Foodstuffs when sold to CPs.

Thresholds for Improving Status
Current# for NextRunning Total
Reviled (Rvd)5 Gold5
Poor (Por)5 Gold10
Subpar (Sub)10 Gold20
Standard (Std)5 Gold25
Preferred (Pfd)10 Gold35
Favored (Fvd)5 Gold*40

[ Best (Bst) can only be achieved if you have at least an Attitude Status of 'Loyal' ]
[ If you achieve Fanatical with a CP, you automatically improve each category by 1. ]
[ Only one Usurper can achieve Fanatical at a time with a CP. ]

Start Sizeto ImproveRunning Total
Hamlet1010
Village2535
Settlement5085
Small Town----

You will not see the change until the next time you visit.  If you scout a grid, in the same week that it would increase in size you will receive a notification in your scout results, "This [size CP] appears to be growing."