What happened to all the grounded Boeing 737MAX ? Will buyers send them back to Boeing and get a refund ?

They are auctioning them off on Ebay. I picked one up last week for $100K and will be flying it around the neighborhood and buzzing control towers around the state. Boeing and Airbus are up a creek without a paddle right now. There have been hundreds of order cancellations and there are perhaps 20K aircraft on the ground around the world, not flying, not earning money.

At present, they are gathering dust.

As for returning them to Boeing, the answer is most likely no, if the planes can be re-certified. Each of those plane is probably covered by a contract that is several miles long, and would spell out in achingly detailed lists the obligations of each parties.

While having a plane that can operate safely is likely a clause that is mentioned, it probably does not describe what can and what cannot be done in a manner that prevents the plane to be operated safely IF that outcome is possible with certified alterations.

That said, there were over 5000 737MAX on order when the grounding occurred, with less than 400 having been delivered, most likely airlines had only received part of their orders, so airlines have a considerable leverage against Boeing. Already, 264 orders were cancelled, and there will likely be more, no only because the future of the plane is not certain (when will it return to service?) and also because the coronavirus has really messed with the profitability of airlines. But that 5% of orders were cancelled implies that 95% of operators are still following the "wait and see" strategy.

The bottom line is that the 737MAX was touted as a plane that behaved so exactly like the 737 NEXT GEN that pilots did not require retraining; and that "similar behavior" was supposed to be thanks to the problematic MCAS system. Remove the MCAS, and you should have a fine plane, except that it will handle differently from the pilot's perspective, and that airlines would have to spend time and cash to have pilot train in simulator; something that will evidently be charged back to Boeing.

Government will convert them into trailers for the homeless.  This means of course, that the tens of thousands of trailers FEMA already bought over a decade ago are to be junked.  Best since FEMA never took care of them.