Let's be realistic. The issue of friends shouldn't even be taken into account as it's largely irrelevant. If you'd take advantage of your "friend", then I suspect you are not really a "friend" to begin with.
As far as items that are priced already . . . once the price is marked it's a fair deal with no guilt. If you want to be a "swell" person and let the store or yard sale owner know, fine be a "hero".
Me, I quietly walk to the counter and buy that
Woodgrain for $10 . . . who wouldn't? If not me, then someone else . . . they've already forfeited the profits as someone is going to snatch that baby up any second. Might as well be me.
We'd all do it . . . every SINGLE one of us . . . and it's best not to mention even a whisper of offering more as you might arouse suspicion.
Hand them the cash . . . walk calmly out the door . . .
And . . . if you feel guilty . . . remember that they probably gave some sorry bastard 50 cents for it, thinking, "Wow, I can sell this for $10!"
"Gleemonex makes it feel like it's seventy-two degrees in your head... all... the... time! "