A} What's the difference between the merchants selling:
1} -<< bronze buckler >>-
2} +<<+ bronze buckler +>>+
I'm uncertain how to even search for such symbols in the forums.
B} Is there some place which provides a key or explanation to these symbols?
C} Why are the merchants able to sell items with multiple symbols when our dwarves creating items\weapons\furniture can only provide one symbol?
The << >> indicate that the item is decorated. So -<<bronze buckler>>- indicates a bronze buckler with - quality decoration, and a +<<+bronze buckler+>>+ indicates a +bronze buckler+ with + quality decorations. You can view what specifically they are decorated with by viewing the item itself. Decorations make the item much more valuable, if not any more useful.
This page is for an older version of DF, and explains the numbers in detail if you are interested- I'm fairly certain the general formula is the same, although the value of individual material multipliers is different in some cases.
You can decorate your items at a forge, craftdwarfs shop, jeweler, or clothier depending on whether you want it to be decorated in metal, bone, jewels, or cloth, respectively. You can decorate multiple times per category, but not multiple times per material (so you can encrust something with a diamond and a ruby, but not 2 rubies, for example.) AFAIK cloth embroidery only works on cloth items like bags and clothing.
EDIT: NINJA!