First off, the sear (DIAS) or lightning link (aka auto-disconnector) MUST be registered. If they are not registered with BATF as NFA weapons (machineguns), they are illegal to use and for the most part, possess. If its called a "pre-81 sear", or link its probably not registered and thus not legal. If its registered, you'll need to complete a BATF form 4, FBI fingerprint cards, get the CLEO signoff, and pay a $200 transfer tax to the Treasury dept. to get it in your hands from an FFL/SOT in your state.
That said, the sear and the link are two different animals. Sears are currently in the $4k range for just a sear - remember the ones you see in SGN for $175 are NOT legal to use, and could get you ten years in the pokey for just possession of the sear alone. Thus, there's not really much savings over buying a registered DIAS (drop in auto sear) over registered receiver M16. Registered DIAS are NOT a myth though - they are available if you know where to look and have $4000-4500 cash redy to go. Same goes for links. The link is a small 2-piece of sheet metal that fits in the receiver and automatically pulls the disconnector as the bolt goes into battery. Both DIAS's and links work well, but DIAS are sturdier and allow for semi auto fire as well as full auto. Links though are cheaper and a bit flimsy, being about $2500. Occasionally they (links) may stop because of bending of the sheet metal trip paddle. Not a huge problem, but annoying when you're trying to rip through a beta in one burst. DIAS's are much more sturdy.
The simplest way though is to get a registered receiver M16. Considering with both the DIAS and the link, you'll need to get a gun to put it in, much of the savings is lost over a complete non-Colt M16 conversion (SGW, PAWS, BM, etc) which are currently in the $4500-5500 range.
The biggest drawback to DIAS's and links is timing - the timing must be on. If not, you could have a dangerous situation on your hands. The other problem with links and sears is that you cannot use a Colt OB LMG kit. Sure you can use the LMG upper, but not in the open bolt mode. Finally, I don't think links will work with Ciener .22 or Colt 9mm conversions (DIAS's and registered receiver M16's do though).