A local auction house wants an exclusive 2 1/2 year deal to sell all of the property the county has to sell, maybe with a few exclusions, with a 15% commission; I assume they meant personal property; I realize LGC 263.152 permits some auction sales, but would an exclusive contract run afoul of Section 26, Texas Bill of Rights?

I've just reviewed a couple of contracts for "ad hoc" sales of county surplus and salvage by 2 different auction companies. Neither charged a 15% commission and both conducted internet sales that broadcast the goods to a wide audience. Neither required an exclusive contract. As for the constitutional issue, I would have to do some research, but... for practical purposes, there are better options. Google Lone Star Auctions, Auctioneer Express (both of which are near you) and Rene Bate's Auctions.