Question

Slaves sold theselves or were kidnapped or something like that ? how were they sold and brought to the market ?

Answer 1

The revenue system during the days of the Delhi Sultanate gave rise to the number of slaves. The subordinate shiqadars were given the task of abducting people for the purpose of extracting revenues. Those who complied were free and those who could not pay the taxes were taken up as slaves.
During the Khilji and the Tughlaq dynasties this system gained momentum, as the slaves were required for territorial expansion and serve compulsorily for the military.