Debt slavery[1] doesn't involve kidnapping (by any conventional definition). It's probably the most common kind of slavery these days.

[1] https://en.wikipedia.org/wiki/Debt_bondage

Slavery doesn't involve kidnapping if there is a legal framework for selling people (which by definition isn't kidnapping).

Are you talking about people born into slavery? Like from slave parents. Because the transition from non slave to slave is some form of kidnapping/trafficking.

The conversion of something to property is almost always a theft of some sort.

I think the Romans could sell their family members into slavery to pay off debts, etc.

Or themselves.

There are people in the South Asian sub continent as well as SEAsia where families will sell other members into slavery for varying economic and social reasons. It involves no kidnapping. Sometimes it's for very menial and manual labor (tea picking, for example, next time someone enjoys some Assam tea) or are later sold into human trafficking rings.