SKU is not available

[naourass]

The seller can't add a product, because of SKU validation error.

Stack : PHP 7.2.x, Magento 2.3.1

[lima195]

Same problem here, you can fix updating this:
app/code/Apptha/Marketplace/Controller/Product/Skuvalidate.php

changing line(72):
echo $skuCount;

to:

return $this->getResponse()->setBody(json_encode($skuCount));

[lima195]

This fix works like a charm with every ajax response in this module