It looks like a Mauser Model 712 Schnellfeuer that's been converted to semi-auto only.
Whether it is an original Mauser product, or one of the many Chinese clones that were made over there in the 20's and 30's, I don't really know.
The Chinese bought many Mausers from the factory, but made many of them right at home. Those latter vary from absolute perfect copys to crude.
Anyway,,the first thing that caught my eye was the trigger. That large bulky trigger is the style used in the 712. The standard C96 uses a more curved and slender trigger.
This 712 style hides a pinned in place 'kicker bar' in the back side of it to operate with the F/A system.
That bar has been removed in this pistol but you can still see it's pivot pin. A standard C96 trigger will not work in these pistols as a replacement.
The frame milling (milled out panels) is the same as on the 712. Different C96 models had different variations of frame milling, some had none.
This one just happens to match the 712 frame.
The detachable magazine was standard on the 712 and not on the regular C96.
To modify a standrd C96 for a detachable mag is doable but the entire mag box must be removed from the frame and a new larger in size box made and welded into place. The larger dimension needed so a detachable box magazine of appropriate size can then fit inside that,
The button latch system then made and fitted and the welds blended and polished, ect.
What is missing is the F/A-S/A switch that was on the left side of the frame on a 712.
That would have had to been removed and the entire area filled iin (welded) and properly machined inside for parts movement. The outside is a more simple flat polish to match the other surfaces.
What it also may be is a later Chinese import C96 (80's, 90's) when they were remaking them and importing them into the USA. A couple different versions were made and in 45 and 9mm cals.
IIRC they did have detachable magazines.
I thought that the 80's imports had different frame panel milling,,maybe not. There's so many variations of it.
The contour of the trigger guard is not the graceful curves you's expect to see on a Mauser. The backstrap edges along the grips in the import marking closeup show rough edges. Again not Mauser type work.
Perhaps it is a later Chinese made import with a few features like the trigger to pimp it up a bit and make it look like a Schnellfeuer.
That's still done with the 'kits' to make a standard .22 look like a M60 or any number of FA weapons.
An orig m712 would be in 7.63cal I think.
A conversion to 9mm is a simple thing though.
Why no mfg markings. The refin looks importer crude enough to be one of the Chinese imports of the late 80's when the flood of C96's can on board.
MAybe the OP just didn't spot them
I'd really wonder about the history of it being a M712 or a copy of one in it's past before I ever bought it.
There's no converting one to a semiauto and being legal w/o Fed paperwork.
Once a MG,,always a MG.
I'd look it over carefully.
It's priced too high anyway IMO.
Here's a pic of a Chinese copy of a M712. It shows the trigger style and detach mag, ect. Kick bar in the trigger is in place in this one.
The reference to 'Wauser' is that many of the hand made Chinese products have hand cut lettering on them as well and in copying the markings the 'M' in Mauser was sometimes inverted and made as a 'W' so you get Wauser.
I imagine most of us would come up with some strange words if we tried to copy the Chinese language over onto something too.
C96 Broomhandle Mauser Model 712 Schnellfeuer Chinese fake Wauser marked | laststandonzombieisland
added..
I think that import mark is :
BTC
S. El Monte CA
If it is,,it's Briklee Trading Company,,,1443 Potero Avenue
South El Monte, CA 91733