Answer:
Dealers profit comes from the spread primarily. Spread is the differential amount between buying and selling.
Explanation:
Let us assume the price of security X is USD 100 (last trade price)
A dealer will purchase this security at discounted price from the investor say USD 99 and will sell the same security in the market at USD 100, thus earning spread.
Further being market markers, dealers often use multiple strategies to prop up the price of particular security and earn gains on inventory held.