Not every item recommended for guinea pigs is safe or good for their health. Here is a summary of supplies that you will want to avoid.
Making your own guinea pig cage using organization cubes is very common. These types of cages are known as C&C cages. If you choose to assemble your own cage be careful to buy materials that have no fewer than nine squares across in either direction (each opening should measure 1.5 inches.) Any fewer and the opening will be large enough to allow your guinea pig to stick his or her head part way through the holes in the grid. Unfortunately there have been reported cases of guinea pigs that have choked and died this way.
Cedar or Pine Bedding
Recently there has been growing evidence that these materials release volatile hydrocarbons which can cause serious respiratory and possibly liver problems. Pine can be safe if it is cured, which means "heat treated" as this will release the harmful agents. However, if you do choose to use pine bedding keep in mind that even though a bag is labeled as "cured" there is always the possibility that the material was not properly treated.
Wheels and Exercise Balls
Your guinea pig needs exercise but not using either of these items. Your guinea pig does not have a flexible spine and both a wheel and exercise ball will cause their spines to bend. This can result in serious injury.
Leash or Harness
It is possible in the pet store you will find leashes or harness that claim to be guinea pig safe. Please do not use these as your pet can be injured when struggling against the leash.
Salt Wheels and/or Mineral Licks
Guinea pigs can over consume minerals leading to bladder and kidney stones.
Pet Store Treats
Avoid any treats with added sugar, fats, dairy, seeds and nuts. Nuts and seeds can cause choking and intestinal blockage. Sugars and fats can lead to excess consumption of empty calories which is not good for your pet's health.
Vitamin C Drops
You will see vitamin C drops widely recommended as guinea pigs are not able to produce any on their own. However these supplements rapidly degrade when exposed to light and gives the water a taste many guinea pigs find unpleasant. If your pet does not like the taste of the water you provide it is very likely that he or she will not drink enough to ensure good health. The best source of vitamin C is still fresh fruits and green vegetables.