You can, however, get unlimited free trials on those sites using a simple trick with your email address if you have a valid card number with expiration date and CSV.
There's also an Android application for getting fake card numbers called Card Gen, available for free in the Play Store.
You can find more about it at: The Walmart Credit Card is limited to Walmart, Sam’s Club, and their associated gas stations and websites.
Also, keeping your account active can help boosting your credit score.
You can generate and validate credit card numbers directly from the app, making it easy to use on the go as well.
Validation in particular would be useful if you were accepting credit card payments on your own site and wanted to make sure the cards were legit.
Now, to be completely clear, these numbers cannot be used to purchase any item. The credit card number generator uses a system based off of the Luhn Algorithm, which has been used to validate numbers for decades.
You can learn more about the algorithm on their webpage, or check out the Null Byte guide on how credit card algorithms work.