Once the crank arm is removed, or if it has loosened up through pedaling, it is normal for the arm to come loose as you describe.
The solution is to simply retighten the crank bolt. Of course, you must have the correct French threaded bolt to go into the crank spindle. You would know this based on whether the bolt was previously okay.

And you must have the correct tool for a complete tightening of the bolt into the crank arm opening.

Not knowing the history of your bike, only you would know this.

I would not hesitate to buy on ebay from a 100% feedback seller of a French 1970's era, crank assembly seller. But, your problem may be simply lacking the proper tightening tool.