Selling X is getting increasingly less profitable

Indicates that selling X is still profitable, but less than it has been and increasingly means the rate it is becoming less profitable is going up.

Selling X is getting decreasingly profitable

Only indicates that the profitability of selling X is going down.

Selling X is getting increasingly less profitable

Indicates that selling X is still profitable, but less than it has been and increasingly means the rate at which it is becoming less profitable is going up.

Selling X is getting decreasingly profitable

Only indicates that the profitability of selling X is going down.