Looking at possibly selling my Gideon Shadow HK UMP 45. Does anyone know of a FFL in the SE part of the state that buys SBRs or will do consignments on one?

Not SE but Brotherhood Arms up in Watertown might.

Walberg Precision up by Aberdeen does a lot of NFA transfers and stuff but I dont know if they buy/resell

Not an easy road....
You would have to form 4 it to the FFL SOT @ 200.00. If an in state buyer if found; the buyer pays 200.00 to transfer from the FFL / SOT on Form 4.
If sold to an out state buyer...the it goes Tax Free on Form 3 to another FFL SOT; and then on form 4 to the buyer with $200 tax stamp.
And, no a FFL SOT can not be possession of your NFA item without being transferred to them properly on a Form Four for consignment purposes.  

The only way and FFL/SOT can be in possession of NFA item is repair or gunsmithing is being done. Transfer on Form5

You might want to advertise in on NFA boards here EE for sale in SD where you do the transfer from seller to buyer with just
one transfer on a Form 4 if an in state purchase is agreed to.  

Pretty sure Gary's Gun Shop does this.