'decrease in custom' vs 'decrease in sales'

Would anyone be able to explain the difference between ‘decrease in custom’ and ‘decrease in sales’.
For example, what would one make of reading:

‘There’s a decrease in custom in our region.’

or

‘There’s a decrease of sales in our region.’?

Thanks to everyone for their replies,

Still hoping of being more helpful to people who’d like to help I’m suppling the context of the phrases in question:

Company directors of a UK energy company are discussing the company’s sales figures:

They aknowledge that ‘sales figures in their region are down’, then ask the Customer Service director:

-(Managing Director): Do you have any ideas why this downturn in sales is happening?

-(C.S. Director): It’s true, we’re seeing a decrease in custom in our region. There’s also an increase in customers cancelling new contracts.

-(Colleague): But do you know why? Do the customers say why they don’t like our offers?

Could the phrase ‘decrease in custom’ be used because the company deal with energy contracts? Could it be because it doesn’t refer to the specific company’s custom but the region’s in general?….