If you ask your friend for $100 for no particular reason, just because you want $100, that's an annoying request and "no" is a reasonable response. It's not putting a price on your relationship. It's technically the same answer they'd give a stranger, but that doesn't mean you're being treated like a stranger.

(I do think a slight discount often makes sense just because a friend is probably quicker and easier to deal with. But anything more substantial turns into asking for free stuff, and yes and no are both perfectly fine answers to that.)

That's the thing. This was a $3000 camera. A 20% friends discount is 600. We've been best friends for two decades, but most days he doesn't give me $600 on cash. Don't get me wrong, we don't keep track who paid for dinner or cinema ticket or whatever. But there IS a threshold at which it really becomes a random cash gift.

Yes dealing with friends is nicer than strangers - but also when you're selling stuff, sometimes it's better to do strangers. Expectations of long term service and support are clearer and have more defined boundaries.