Update:
To replace the $ -> € sign in "Settings -> Membership Level -> "Level -> Pricing"
change the values in table "sys_options":
'currency_code' -> EURO (maybe EUR ?)
'currency_sign' -> €
'available_values' - €$
Then there is also the € sign for "Pricing for '......' Membership Level"
Example: 365 Days - 30 €

Using these settings, either EURO and EUR, I did not get the error message "The vendor see more does not accept payments" . I was directly forwarded in "Test Mode" to PayPal with a link for the PayPal Sanbox!