For any of this to work you will need to define item worth for selling or for buy. This can be done with /cmi setworth which will open UI where you can define prices

Sell hologram

Basic example of hologram which not only shows items worth, but indicates it with correct icon and can be clicked to sell item from your hand.

Entire hologram section should look like this

shop:
  Loc: LT_Craft;-102.5;74.12;86.29
  Interval: 0.1
  LOSInterval: 1
  Range: 8
  RangeExtra: 16
  Spacing: 0.25
  SpacingIcon: 1.0
  Interactable: true
  Commands:
  - check:%cmi_iteminhand_worthc%>0#!
  - hasitem:%cmi_iteminhand_type%-%cmi_iteminhand_amount%?#! asConsole! cmi msg [playerName]
    !{#cancan}You have sold &2%cmi_iteminhand_realname% {#cancan}x &2%cmi_iteminhand_amount%
    {#cancan}for &2%cmi_iteminhand_worth%
  - item:%cmi_iteminhand_type%-%cmi_iteminhand_amount%?#! asConsole! cmi money give
    [playerName] %cmi_iteminhand_worthc%
  Lines:
  - '{#Cancan}Sell'
  - ICON:%itemInHand%
  - '{#Cancan}Worth: {#edward}%cmi_iteminhand_worth%'
  - '%cmi_iteminhand_realname%'
  - '{#gray}Worth of one: %cmi_iteminhand_worth_one%'

Simply copy/paste this entire section into hologram.yml file and update its location values to fit your needs. You can always update it more accurately in game

Sell Sign

Similar approach can be used for signs.

  1. Place a sign
  2. Look at it and perform /cmi dsign new
  3. Add 3 text lines to it, so it will look like this
  4. Open sign UI by clicking on and adjust update time to 0.1 for more fluid experience

First part is done, you should see sign changing its text when you select different items in your inventory

Second part is to create interactable command. Simple perform /cmi ic new sellshop

This will open similar interface. While looking at sign we just made click on first green + sign which will add this block into the list. Next would be to switch into command editor which can be done by clicking on ! sign then click + sign and you can enter commands. Commands are exactly the same as for holograms and it should look like 
 
And you are done. Sign should update with information how much you will get when clicking on sign and what you will sell.