The Wilmington ATF Field Office (Criminal Enforcement) must be short on work, looking for low hanging fruit. One would think that even in Delaware, the local PD’s would recover enough crime guns to refer to ATF for traces. Those successful traces to local FFL’s/buyers, would presumably give them plenty of leads to follow up to assist the local coppers. Again, using these intimidation tactics are poor business and the hallmark of mediocre agents, and maybe in this case a group supervisor or Resident Agent in Charge.
“Good afternoon. Do you have a warrant?”
[some form of response that includes the answer NO]
“Thank you for stopping by, Agent Bob. You too, Deputy. Goodbye.”
[Close door on them.]
All done as pleasantly and smiling as possible.
Best not to get all fierce and lecture them on probable cause, due process, 4th amendment, or anything else. That will just piss them off and likely cause them to look for an excuse to roust you further.
And yes, if you have a front door security camera (Ring, etc.) preserve the recording.
You may wish to use it when writing your Congress Critters asking them to rein in the ATF.
A good outline of what to do and what not to do. 
