Show products based on custom field
Depends on whether the user field is on the product or a variant.

If the custom field is on the product:

{% set products = craft.commerce.products.relatedTo({
    sourceElement: currentUser,
    field: 'userFieldName'
}) %}


If the custom field is on the variant:

{% set variantCriteria = craft.commerce.variants.relatedTo({
    sourceElement: currentUser,
    field: 'userFieldName'
}) %}

{% set products = craft.commerce.products.hasVariant(variantCriteria) %}