Quick Trade was created for the purpose of converting one asset to another instantly. When you enter an order in Quick Trade, you will be given a quote before deciding whether you wish to execute the trade. Depending on your review of the provided quote, you may execute the trade, or cancel and make another trade.
For Quick Trade to execute in an instant, the function will purchase your desired currency at the lowest possible 'sell order,' a.k.a. market price. If your liquidity volume is too large for a single order, your Quick Trade will compute continuously down the list of open sell orders until the volume requested is purchased. These trades will eventually increase in price as you go down the list (as sellers want to sell for higher prices). Quick Trade, however does not factor in market spreads, which is the difference in the price of the sell order and buy order. These spreads, when extremely high, will directly affect the amount of currency you will receive, as the market price will differ from the lowest sell order.
If you are converting any currency other than Bitcoin (BTC), a two-step transaction will occur. During this two-step transaction, the market price of both BTC and the currency you are buying will be involved in the computation of the trade.