If the student loan was bought by a collection agency are you still liable to make payments?

Yes, debts are assignable.

Yes, why wouldn't you be? Just because a collection agency has it does not mean you are
Free from paying it.

If they will accept payments, and not merely wish to go for a garnishment or attachment. But it is best to work out a payment plan with them if they still will.

Basically you are liable for paying off a debt until it is paid off. Nothing but bankruptcy or settlement can change your obligation to pay in full. Debts can be sold, transferred or assigned for collection. None of that changes your obligation. Doesn't matter the type of debt as long as the debt is valid (you agreed to pay it) then you owe it.