That would be due diligence, which wouldn't be a necessarily good defense in a private gun sale/purchase where chain of custody wasn't established. If you don't know the source, then be suspicious. Still better to err on the side of caution I would think.

Most likely it is a non issue 99% of the time, but for me, it just doesn't seem to be worth the potential hassle that could come of it. One-off transfers wouldn't be bad, but if I am the third or fourth party out, I would start to get a bit paranoid, and be wanting that identification of transfer from the previous possessor handy should the need arise.

In some jurisdictions, it is just easier for the LEA to arrest you and leave it to the DA to deal with it. They can arrest you, charge you, take you to trial and you can be found not guilty, but you will still have a record (flag) with NCIC that could be a real problem with all future fiream purchases, etc. This admittedly is walking into the "What if..." realm a bit, but it can, and does happen.