This vendor is not accepting payments

I added a product test. Testing using sandbox paypal. I keep getting a "Vendor does not accept payments". I've been working on this for 8 hours. I've read all the documents. I've added a product Test1 through the front door as an admin. Created an account test. The when I go to ad the product to the cart that's when I get the above message. I've double checked the sandbox setup, I've checked the paypal set up. I totally lost. I'm using version 7.3.3 

Finally got the vendor issue fixed, but now the store will not add the item to the shopping cart. It states when test member selects the item to be added to the cart, but when you select the shopping cart it shows it as empty. 

Hello godjar, can you just explain how you've fixed the vendor issue ? I've the same Problem (Dolphin 7.3.0 and Ubuntu 16.4 with php7.0

And probably you know answers to two other Topics that puzzel me:

1) What shall I fill in in the "Embedly key" under the payment method in advanced Settings?

2) Is there still a member Action like "add product to cart" in 7.3.xx ?? in the Membership Levels?

Thanks in advance

To accept payments vendor (author of the product) must fill in payment details in Accounts > Payments > Settings. The payments specified in Admin panel > Modules > Payment are for selling membership levels.

1) "Embedly key" has no effect payments, it's completely different

2) there is no such action, however there is "store view product", so anyone who can view product - can add it to the shopping cart