Local guy has one with 4 mags and a HK zipper bag for $1150.  Is that a smart buy?  Looks like it'd be fun.  Does it share mags with any HK 45 pistols?

A quick gunbroker completed auctions shows new ones selling for 1300-1400 and some used selling for slightly less.  I’d say your guy is offering a fair deal.  I do not believe ump 45 mags work in anything else other than the LWRC SMG 45.

But are USC and UMP mags interchangeable? I thought the USC was a single stack.

No I guess not.  Sorry I just assumed they used UMP magazines, sorry about that.  Looks like you can have tommy built do a UMP conversion on your USC though.  That’s probably what I would do if I owned one.

USC single stack mags are not compatible with any other gun I am aware of.  

The USC is a fun gun and relatively quiet.  Shooting a subsonic round out of a 16” barrel is not loud.  The recoil is a bit jumpy, but that doesn’t detract from the fun factor.  

The price seems in line with market and a fair deal.

Good deal, FO at that price.

Always wanted one of the gray ones, that seems like a fair price.  I would FO if you are so inclined

I once saw on that was converted to use Grease gun mags.  The owner told me there was some gunsmith that did the conversion.

I think that was the main mag used in a lot of early conversions. UMP mags were unobtainable for a long time. I remember people getting ahold of LE UMP mags and they sold for crazy money.

I have a USC converted to UMP. Cost of entry is cheap, cost to make it cool is far from cheap. UMP parts have also gone up substantially in price from what they used to be, I used a stock block for conversion rather than the plastic weld.

They used to sell grease gun lowers.  The USC upper has to be cut to accept the wider magazine, similar to doing an UMP conversion.  These lowers use a different mag release that’s compatible with grease gun mags.

I had one and traded it in.  Many better pcc options out there.  The way HK castrated the USC makes it boring in a hurry.  
Unless your end goal is to send it off to TommyBuilt tactical for a full UMP SBR conversion and spend another $1200 at least for all that, then forget it.

There ARE actually other options, depending on the extent you want to A) spend money B) do some work yourself

This guy modified his stock using a heat gun/plastic weld/some strategically placed screws, giving the otherwise unmodified rear end a much more UMPy look. You can also buy 15rd ProMag brand mags for the USC that are the same length as the actual 25rd double stack rounds, further giving the appearance of the converted ‘real deal’. I know I know, ProMag sucks but the general consensus on these 15s is that they are surprisingly reliable.

His has also been SBRd, which adds expense, but you could certainly skip that step.

Any combination of the above mods gets you significantly closer to the UMP look while keeping cash outlay to a minimum.