Many of the sellers there are either selling: RDP, Gift cards, accounts and microsoft keys.
RDPs can be easily hacked and many are sold this way. Guarantees are offered as many would reinstall the OS resulting in you unable to access it again. Guarantee means that a replacement would be offered to you if it is gone before the guarantee is finish.
Many gift cards are bought from stolen credit cards, i.e.
Carding. It is easier to hide from the owner than buying Bitcoins directly. Hence, the lower pricing. Some offer guarantees too, to reduce the risk, you can ask them to take a picture of the card and the receipt.
Accounts are usually either hacked or trial accounts. Companies like spotify offer trials for one month and people make use of this to sell their accounts to others. Hacked accounts are quite prone too. Some may make use of carding methods to pay for the account.
Microsoft keys are usually obtained from MSDN. They are not meant to be sold, rather for personal uses. If they discover it, your keys would all be invalidated.