so how would I keep the parts in without the pin??
The bottom edge of the upper is rounded. Install the buffer spring and buffer and hold in place with your thumb. With the front pin in place, just rotate the upper down until the buffer is held in place by the upper. REMOVE finger, then close completely and put in the rear pin.
When you remove upper, same thing but in reverse. Pull pin, rotate upper away from lower about an inch, INSERT finger to hold buffer, rotate upper all the way up, pull out buffer.
It is actually just as easy as with a buffer retainer and maybe even faster for me. I run all of my ARs with no buffer retainer now.
A magazine block in the vice or a magazine block stand does help a bit.