Another point of consideration is that there is a reason the unit is for sale.
Usually an item is not for sale because it is working fine and will continue to do so for a long time.
There are several flavors of used BMAs floating around out there.
Some really good ones that function just fine and will probably stay that way for several HUNDRED (not Thousands) of hours.
There are some early Gen 4s that were never installed nor updated at the factory for known problems. You really do not want one of these. Then there are those being replaced because they have issues. Avionics issues rarely get better with use and never fix themselves.
There is virtually no price that will make these units cost effective.
I loved my BMA units . Over the years I have owned 5 of them but now they are obsolete. If you put one in your panel this is what your panel will look like soon after when you have to replace it.
Now you have one huge hole to fill and that ain't cheap. Guess how I know.