Payday lenders will first start calling you all the day. Threat of an arrest, legal charges for check fraud and so on. They will call at your work, try to withhold your wage using the power of voluntary wage assignment (which is often authorized by digital signature, hence invalid), call your references and so on. Then they will send it to collection. Now collection agency will start doing the same thing.
Now most of the payday lenders do not report to none of the EQ, EX or TU, they report to Teletrack. But if your lender sells your account to a debt buyer they can report to major credit bureaus and non-payment will hurt your score.
I have heard that store front lender sued debtor, but never heard that an online payday lender brought legal charges for being default. Moreover, there are doubts if the online payday lenders are licensed to do business in your state or not. That could be the reason they refrain from legal activities.
However, if you owe the debt, try to pay as much as you can. All the states have a certain usury law. Follow it. If your lender is charging interest or fees more than what your state permits, file complaint against them and pay what you lawfully owe.
Browse this forum, search for the posts of
The Payday Loan Avenger and
pollyandsay. Both of them have done extensive research on online payday loan laws. I'm sure you will find it useful.