You can follow the instructions given below to insert user id as product SKU.
1) Add a hidden field to the product form you created & insert _sku meta key to that field.
2) Now, Add the following code inside the theme’s functions.php file.
GeSHi Error: GeSHi could not find the language php (using path /home/mahbktbn/public_html/wp-content/plugins/codecolorer/lib/geshi/) (code 2)
All done, submit a product using frontend product form & check the product SKU in wp-admin. The SKU id will be the user id.