A Sheriff may question the legality of the Federal agent's action on complaint by a citizen.
Yes, they can.
What you are referring to is a longstanding "policy" of noninterference.
POLICY IS NOT LAW.
A Sheriff is the highest ranking law enforcement officer in his/her County -- Full Stop.
If a Sheriff smells crap around a federal agent, the Sheriff can stop said agent until they appear in Court before a Magistrate.
The Magistrate in turn, can question the legality, validity, veracity of the federal enforcement action and tie it up until all concerned have had their say.
VALID federal actions are backed by the Supremacy Clause. INVALID actions are not. The determination of a challenge to validity of an action is under jurisdiction of a court.
A federal court may try and send Marshals to clear the way for federal agents, but the Sheriff can delay them too.
All of the above is an expression of the People's elected law enforcement representative acting on their behalf. It's the People saying to foreign entities, "You will not cross this line until we are persuaded." The Governor can refuse to activate the National Guard against the citizenry of the State. A President cannot legally call up the Army to invade a State.
Thus, the Sheriffs can cause a standoff that culminates in hearings before a State Court.
Try as you might to dismay the People, but if they stand firm, you have no recourse because they can be deputized and armed, and there are orders of magnitude more of them than you.
Think it will never happen?
Try your luck. You have maybe seen a few hotheads and thugs act against your orders, but you have never seen the response of an entire People pushed up against a wall reacting in ways you never imagined.
The fellow above had it all, the police, color of law, the military, the press, all of it. He thought the People rising up to overthrow and execute him, that was never going to happen. He found out, didn't he?
The play is going to be with the Sheriffs. They are elected and must answer to the people. They might ignore the crowds for awhile, but eventually they cannot ignore and avoid their County’s share of 100+ million people united against tyranny.
There will be Sheriffs falling in line with the People that elected them. They are not going to listen to some nobody tell them what the rules are. They are going to use any PRETEXT to stop, delay, frustrate federal agents from carrying out illegal acts UNDER COLOR OF LAW.
We are there. Civil War. The line has been crossed. You've never seen it. But you're about to. Normal passive law-abiding people that have never had even one infraction are suddenly going to be looking at you. They are going to be preparing for things you never thought possible.