Many times we see "parts guns" made on our stripped lower receivers using less expensive inferior aftermarket parts from unknown sources, barrels are not marked for instance, and people sell them as complete Bushmaster rifles so the value is not there. We have seen parts such as plastic buffers full of lead shot, plastic delta rings, plastic mag catch buttons, A3 handles with unmarked one piece elevation wheels etc.
If someone uses all Bushmaster parts to build a rifle the value is usually the same as factory rifles.
Sometimes values are increased when quality parts are used to customize the rifles such as Picatinny handguards, pistol grips, buttstocks, back up iron sights, optics, competition triggers etc.
The cost of saving is not as much as it used to be to build your own.