Can an SBR be sold or transferred in MD as a regulated firearm (obviously through State Police or FFL)? If so, does the approved Form 1 need to be transferred with the SBR or does the new owner need to get a new Form 1? Does the seller (builder) need to keep anything other than the copies of the Maryland paperwork?

it must go thru the batfe

Scotty is right, it's a federal issue and has nothing to do with MD's (local) "restricted firearms" legislation that pertains to 'assault' rifles and handguns.

Your approved Form 1 only entitles you to build/posess the SBR and doesn't get used for the transfer.

To sell it, it will transfer on a Form 4, just like a suppressor or MG. The buyer pays the $200 tax,  gets the CLEO signoff, provides a photo, prints, etc. . .

The MSP's only involvement will be to approve the buyer's Form 1 paperwork so it can then go to the BATFE. There is no specific "MSP paperwork".

Most people who build SBR's on AR15 lowers and lose interest usually just sell off the upper, strip the lower (or just take the stock off) and put it in the safe to sit. Takes up very little room and is worth far less than the tax paid to transfer it. The Form 1 doesn't expire and doesn't cost anything to keep . . the registered lower can always be reactivated later if you change your mind.