Looking on gunbroker and ebay, sellers are saying their bolt carriers are chromed on the inside where the bolt is inserted, to prevent wear, and only good expensive manufacturers do this. is this a selling scheme? are all carriers from "big boys" iike this?

Watch it Troy - some of the members here get upset by the term 'Bottom Feeder'. [;)]

Ok, now specifically speaking, what about DPMS carriers?

I've seen them use chrome on the outside...Surely there must have been some leftover for the innards.

Uncrhomed carriers are a feature of the low end guys.  You can use it as a gauge of quality or to sort the basement builders from the real factories.

Other gauges might include:

1.  Forged receivers versus castings (castings often have raised lettering instead of impressed) or receivers made from cruder extrusions or slabs.  
2.  Chromed barrels.
3.  New versus used parts
4.  US vs imported parts
5.  Military and export sales record. These customers buy only from the top companies.  (Don't consider police customers because they tend to focus on low price.)
6.  Real factories vs parts assemblers.  Assemblers tend to buy the cheapest parts and don't have much control over quality.
7.  Companies with real QA departments.  I've visited a couple of small companies I won't name and they don't have ANY quality control.

The inside of my Bushie's BC is chromed. It's so shiney..... :-p

Colt
Bushie
Rock River
DPMS

Those are the ones I have personally examined.  I'm sure there are many others as well, that I haven't had the opportunity to see[:D]

My Oly bolt carriers are chromed inside, and stamped with the Smith Engineering logo.  My Armalite is chromed inside.  My three DPMS's are chromed inside and out.  Even the J&T is chromed inside.