Seller has a rights to ask to see or sign stuff. There are nothing wrong with it.
You can ask me to provide strain of DNA sample, drug test, proof of citizenship/birth certificate, Green Card, CO ID, CCW ID, business card, resume, school transcript incl GPA, etc. "If" someone is selling an NIB $10,000 firearm for $100, fine.. I "might" do all that.
If someone is selling $600 firearm for $550, probably would not jump all the hoops that seller requires.

Just that it wont really protect you from anything. I really don't care about doing/not doing the bill of sale.
Worst case, if i do need their info, I already have tons of PM/email showing that I Bought/Sold the firearm. It is my/their firearm now.