Customer-supplied bits
Of course, when the owner supplies avionic parts to his friendly engineer to install ( and hopefully correctly certify with the right EASA approved mod paperwork), he will doubtless expect such engineer to investigate any subsequent failure. Where that failure is down to equipment rather than an install fault, friendly engineer has incurred investigation time with no recompense, probably has to send parts off for warranty - even new bits don't get warranty freight paid, and all without making a buck on the parts.
You wouldn't go to Kwik-Fit with your own exhaust and expect them to fit it would you?
And of course, we assume owner has sufficient savvy to buy the right bits in the first place with correct paperwork. Naturally he will have obtained quotes rom several reputable avionic shops who have spent time coming up with the appropriate interfaces, described required adaptors that may be needed etc, and all for no reward.