Buying at 5-15% and selling at 30-40% of CV is easier than it sounds, and who doesn't want that kind of margin? It doesn't mean buying large collections and ending up with remaindered dross you can't get rid of either.
Here's an image from an online seller that I've purchased from, and who I trust. He regularly creates bundles of stamps that fall outside his sales criteria. In this case Gibraltar QV, one of my favourite areas, and a constant seller.
(A quick look shows one space-filler (25c ultramarine with a tear), which he doesn't include in his stated CV)
Then:
SG1 overprint on Bermuda mint (CV £23)
SG 8 ½d Dull Green x 2 (CV £20x2 = £40)
SG29 75c Olive-green mint (CV £35) and used (CV £32)
SG30 1p Bistre used (CV £23)
SG32 2p Black and Carmine mint (CV £15) and used (CV £35)
That's 8 stamps out of 17, ignoring the space-filler, and not counting the others, with a CV of £203. His asking price is £25 including postage! That's easy money for quality, in-demand classics. There's no returns on these items for obvious reasons, but he hasn't let me down yet!
If you want his link, you're going to need to comment on this post and I'll message you