Bill of Sale

If anyone noticed the thread, I posted a little while back about some Mutoscope cards I have. I asked for some info about them, and that turned into someone wanting to buy them.
As it turns out, these are valuable. To the tune of me facing a sale in the area of $6000, maybe more.
The person is requesting a bill of sale for the transaction, and I think I should have one too. He will likely be paying in cash.
Does the state of Wisconsin require a bill of sale for a transaction like this? Would this be something the IRS may want to see come tax season? We're going to be meeting up in Illinois (about halfway between us) and making the sale there. Does this mean it will be subject to any laws in Illinois governing this?
I can't find infomation online about this. I am also wondering if I can just type up a bill of sale, or if I am better off downloading one from a pay site for this.
Am I just overthinking this, or do I need all these questions answered?
Any help would be appreciated.