How do I save tools within a players inventory?

Is there any possible way to save tools in a players inventory, so that when they rejoin the tools they previously had before leaving will remain in their inventory? For example they buy a tool from a shop, then leave. I want to make it so that the tool will remain in their inventory when they rejoin. I know that there was an old way to save tools, but that was deprecated, and I have no clue how to save tools now.