The TT value is represented by the nominal value carried by an item consisting of all component values included. This is the value that the game has atributed to an item. You can sell an item to a Trade Terminal and get the value the item carries.
However, as a player has to offer a competitive price compared to the Trade Terminal in order to buy an item, there is an extra markup on this nominal price consisting firstly of a demand markup, which is based on the supply/demand ratio for that item, and a luxury markup, which is the amount that a player might pay extra to get hold of a specific item quickly. Additionally, all markups EU vary in time, place and other circumstances, similar to how the market in the real world works today.
The expression TT is also commonly used to express the price of an item, either as tt+price or a percentage of the TT value of the item.
You are selling a Jester D-1 with TT value of 1.5 PED for 2 PED. The selling price of this item would be TT+0.5. 0.5 is the markup on the TT value, which can also be represented by the difference between the TT value and selling price. TT+ is used when selling a single non-stackable unlimited item.
You are selling a hundred units of Force Nexus with TT value of 1 PED for 2 PED. The selling price of the Force Nexus would be 200%. 200% is the percentage of 2 PED over 1 PED, or the selling price over the TT value. The expression 'TT' can be suffixed to 200% (i.e. 200% TT) although this is optional and entirely up to the seller. Percentage is used when selling stackable or limited item.
Usually items that have a full TT value and can decay, have a tt+ value on them. Items such as resources that do not decay and are not directly ‘used’, have a percent showed as the percent of the new price over the TT one.