Some say that you buy the three best in the barrel and assemble one worker and have a bunch of parts. They also have tables upon tables of spare parts for all kinds of guns but you I guess need to know what your doing which when it comes to military rifles I admit to being not that informed. Kind of a strange place. I assume that you would need to take a head space gage with you?
I've bought parts from Sarco for over 30 years, first through the mail and then on-line. Never did visit them while I lived in NJ, though. The Easton part is a relatively new adjunct to their operation, so they could get out from under increasingly-burdensome NJ restrictions on magazines, 'Assault weapons' as defined by NJ law and their parts, and just the problems of selling firearms in NJ generally.
If the SN matches on the bolt and receiver, and it seems to have the original barrel and has a used look to it, it was correctly headspaced for whatever round it shoots long ago. I can't speak for license-built guns, but if Mauser built them they should have the last two numbers of the SN on most of the major parts. Guns that were never issued may be reserve weapons that are short-chambered by about five thousandths, but those kind of pristine cherry pieces are not what you're gonna find in the sort of barrel lot you're talking about. Other than re-assembling the inner guts of the bolt, there is nothing difficult or even complicated about putting parts on a Mauser. The people at the store would be able to sell you the right parts, if they have them in stock, to replace missing parts, none of which are all that expensive or hard to find, aside from possibly the firing pin assembly out of the bolt. There's a special tool for installing and removing the extractor, but installing one can be done fairly easily without the tool. If they're short on helpfulness you could take a tablet with you and figure out the relevant part numbers from the Sarco website once you identified the model, then shop for those specific parts at the store.
Really wouldn't need the headspace gauge unless you bought a rifle without a bolt, or a mismatched bolt, but buying a boltless one for anything but a parts gun on another build would be rash, since a complete bolt might be as expensive as the rest of a beater rifle for some models, and even with a primo unmatched bolt there's no guarantee it'd headspace properly.
