Pricing the rifle is near impossible, I have recorded no end results for a similar rifle at auction (Oberspree/18 or 19). You can't compare this rifle to a normal G98, for one it is far more rare than the typical rifle on Gun Broker, second, this rifle is probably in a similar condition it left the factory, - stock damage aside, the rifle almost certainly had this style rearsight when made, though it may have been a dual compliance with slightly different markings. (in otherwords it is far more original like this, blued with this RS, than your run of the mill upgraded G98 encountered on GB)
Secondly a near matching Oberspree rarely comes up for sale, they were only made in 1918 and 1919 and in very small numbers. Most you see from this company are WOK's and they are fairly common by comparison.
You do have some challenges though, the stock damage is a serious distraction, second only a handful of collectors know what this rifle is, and most that will be bidding will not realize this rifle was never "reworked" postwar... most will think it a normal wartime G98 that was reworked in the 1920's, which generally is a distraction. (reworked rifles are far less desirable than their original counterparts, but yours is pretty original)
I would say its worth about $800 or so, but realistically you have to find someone with both the knowledge and the means. Not so easy on GB, - where most buyers are imbeciles who need flashy lights and grotesque displays of German militarism to take interest in a German rifle.
Never heard of Gunboards appraisals, but anything to do with Gunboards is generally a cause for concern; the collective intellect of all of Gunboards-98k couldn't power a lcd wrist watch, needless to say answer a question that didn't involve plagiarism... Companies do not do appraisals, people do, and you need specialists to do them. I know pretty much everyone who knows anything about German military rifles, especially Imperial and interwar rifles, and I doubt anyone behind the appraisals knows anything about the subject, - perhaps some gifted parrot that will regurgitate something found on this forum, but nothing developed through independent research.
If I put this gun up for auction, any suggestions as too any minimums, or what would be reasonable to expect?
Or in the alternative, has anyone used Gun-board's appraisals? If so, what do you think of that service?