I never had a lumber mill so I used my trading post to get Garrison resources by handing in ore or herbs. Now that I have capped Garrison resources I trade my Garrison resources to the vendor for trade goods or gold.
I noticed the other day that the price of things was different from day to day in my Trading Post. I had vaguely noticed that the trader was different on different days but I decided to look closer this time. I only ever buy Draenic Dust because it sells the best on the Auction House compared to other items. One Draenic Dust sells for about 3g give or take a gold depending on the market fluctations. If you consider that a Smuggled Sack of Gold for 50 Garrison resources gives you 12-17g then Draenic Dust is pretty good value.
Pyxni Pennypocket is one of my favourite vendors, because she sells what I want to buy at cheap prices!
She sells 5 Draenic Dust for 20 Garrison Resources as well as 5 Herbs for 20 Garrison resources. So I stock up a lot when she's on site!
Yesterday I had Elder Surehide, an orc female. She sells Draenic Dust for 25 Garrison resources, so not as cheap as Pyxni, however, she does sell Sumptuous fur for 20 Garrison resources and Ore - not that I need to buy those.
Ribchewer, a male Laughing Skull Orc was with me earlier in the week. His cheap goods are Raw Beast Hide and all the fish flesh for 20 Garrison resources. His Draenic dust is 40 Garrison resources so I try not to buy from him.
Taitasi, a female troll, sells just about everything for 30 Garrison resources (except the Smuggled Sack of Gold). She's a middle of the range trader, and I don't tend to buy from her.
So spend your resources wisely if you're capped! Make sure you get your best value for Garrison resources with your vendors. Oh and by the way, you can only see the vendors if it's your Garrison - you can't go to other people's garrisons and trade with their vendors!