Welcome @mowolf
Sitters are not allowed to ask for money on THS . This is what the terms of service say
5.3. The following applies only to Sitters. You will:
5.3.1. not require or demand any form of payment for carrying out a Sit
If a sitter demands / suggests that you need to pay them - can remind the sitter of the T&Cs or you may want to report the sitter to THS
E-mail support@trustedhousesitters.com
There are many reasons why a local person may do an unpaid THS sit . This has been discussed on previous threads like these 
https://forum.trustedhousesitters.com/t/sitters-do-you-sit-locally-why
https://forum.trustedhousesitters.com/t/some-benefits-to-a-sit-close-to-home
A host can give a sitter a gift of appreciation but sitter cannot ask for or expect anything.
There are many sitters on THS who also do private paid sits , but these are arranged outside of THS platform .