Is it a 712/60, 712/80, or a 712/100? A faster one would increase the value to some degree. Also, the 712/100 has 6 memory slots instead of the 4 memory slots of the 712/60 and 712/80.
Probably isn't much worry about battery damage. They use a coin cell battery (a 2325 instead of the more PC common 2032) and those generally aren't much of a leak risk.
At slight risk is the now old and brittle plastic case, even if otherwise in new condition. If handled roughly, some of the tabs which lock the cover in place can easily be broken.
In general, there are two classes of possibly interested buyers. Primarily hobbyists and collectors looking for cheap deals, and then for some HP gear, businesses which have some legacy solutions built around old systems and who on occasion need exact replacements to keep their operations going. The second class would be willing to pay a lot more, and that explains some surprisingly high sold prices which occasionally show up on eBay, but you might wait a very long time for one of those buyers to come along for any specific bit of HP gear. There are some companies which focus on that second class of customers, for example
http://www.cypress-tech.com/ If you were in the US, you might have even been able to sell a brand new in box system to Cypress, but obviously for a lot less than they would turn around and sell it to one of their own customers.
For what it's worth and as just a wild guess, to a cheap hobbyist, an HP 9000 712 in brand new in box condition might be interesting somewhere around the $200 - $300 range, or maybe slightly more to a hard core collector where the brand new condition is worth a premium. Of course if they have to pay a lot for shipping, especially for the monitor, that reduces what they might want to spend on the items. I have a handful of HP 9000 712 boxes that I probably paid somewhere around the $100 ballpark each, but that was several years ago and certainly not in brand new in box condition.