OOP:  Out Of Print or Out of Production (this doesn't always mean that they are "rare" or unavailable elsewhere--I found a CD at CDNow for $10 that was selling on Ebay for $45 as "OOP").  It can also mean "out of package"--be sure to ask which the seller is implying.
 

BB:  Some people use this in describing the round BB sized hole drilled or punched through the corner of a "cut-out" album.

COA:  Certificate of Authenticity (in reference to an autographed item).

COC:  Cut Out Corner.

DBX:  The records were mastered to boost the dynamic range on the record so that when it played through a DBX system it cut the output, helping to eliminate all those pops and crackles from surface noise.  It will play on a regular turntable, too.  Actually a nice find and somewhat of an oddity, not unlike Quad disks.

DG:  Dynagroove.

DJ:  Disk Jockey Copy (A promotional record given to radio stations).

EP:  Extended Play (records, video tapes).

FS:  Factory Sealed.

HTF:  Hard to Find (may or may not be true.  If it's for a CD, check around at some of the online music stores before you start bidding).

LP:  Long Playing Record.

LSW:  Label Shows Wear.

LTD:  Limited Edition.

M:  Mint or Mono.

MCU:  Might Clean Up (might be a higher grade if cleaned).

ND:  No Dog (On an RCA label).

N/R, NR, NO/RES, NORES: No Reserve (the seller has no "hidden" price).

OOP:  Out Of Print, Out of Production (sometimes "out of package).

OST:  Original Sound Track (music from a film or musical).

P, PR, PC:  Poor condition.

PD:  Picture Disc (The record itself has pictures/images on it).

PS, P/S:  Picture Sleeve (pic slv, etc., depending on how much room there is in the title).

PIC:  Picture (may mean picture sleeve or disc, but probably just noting there is an image included of the item for auction).

P/O:  Punched Out (inventory that has a hole punched through it--see BB).

POPS:  Promo only picture sleeve.

Quad, Q:  Quadraphonic recording  (A remastered album, similar to DBX.  These were produced between 1972-1977).

RI:  Reissue.

S:  Stereo.

SC:  Saw-cut:  About a 1 inch slice (cut out) near one corner of the album jacket.

SD:  Shaded Dog (On an RCA label).

SS, S/S:  Still sealed in factory wrap.

SCR:  Scratch.

SCU:  Scuff.

SLT:  Slight.

SOL:  Sticker on label.

SR, SRW:  Slight Ring Wear.

ST:  Sound Track (music from a film or musical).

SW:  Slight Wear.

VHTF:  Very hard to find (may or may not be true--search around before bidding).

VG:  Very Good condition.

V/M/D:  Visa, MasterCard, Discover (seller accepts charge cards).

WD:  White Dog (On an RCA label).

WLP:  White Label Promo; usually is a plain white label on the record or plain white cover.

WOC:  Writing on cover.

WOR:  Writing on Record.

WRP:  Warp.