Hmm... like Ryan, I too have hacked up quite a few rifles for custom builds, some of which were quite expensive. Here are a couple of the pieces that I have cut up which tend to fetch a pretty price loose:
HT SIG556 - hacked and heavily customized to make it more accurate to the 1:1 version.
HT EBR (x2) - telescoping M4-style stock hacked off one for use in another project (see below), remaining receivers hacked up and cobbled together to make M14 Juggernaut Rogue bullpup chassis (WIP).
TC Mk.14 Mod.1 - cannibalized for parts: flash hider used in Colt SCW, stock and grip used in tactical shotgun project (WIP), upper handguard used in M14 RAS project (WIP).
SSC XCR Micro - hacked and modified to make it more accurate to the 1:1 version, and to straighten the stock.
SS M4 CASV-EL - removed stock and buffer tube and replaced it with a Magpul M93a stock hacked off a HT M4. Also scratch-built a flared magwell for the CASV.
HT Noveske Diplomat - removed flash hider for use in another project, replaced stock and shortened buffer tube to accommodate it, hacked up the handguard in order to remove the Surefire grip which was VERY well glued on and replaced it with an AFG-1.
DAM M870 - pistol grip hacked off to make way for telescoping M4-style stock (stock taken from one of the HT EBRs mentioned above).
BS FN FAL - hacked and heavily customized into a SA-58 OSW (WIP).
PH Mk.18 Mod.1 - stock and buffer tube removed and replaced with CTR stock cannibalized from a SS M4 (M4 receiver broken in the process).