Well, it's hard to say anything for sure without both seeing the horn in person and knowing the geographic area you are in--prices in the industry have no real standards, and fluctuate wildly depending on where you are.
My first and most important recommendation is to take it to your local repairperson. If they are worth their salt, they will assess the instrument's condition honestly and for free. Not many of us are skilled at valuation for sales, but they should be able to tell you what it needs and how much it will cost. Try to get someone with at least a decade of experience in woodwind repair, if possible.
The Mark VI is a quality horn, and will likely fetch a decent price even with a broken/missing spring (so long as you are up-front about it, of course). I personally would get it repaired before selling, though. If your assessment is pretty close and it only needs one spring replacement and some regulation work, it should cost very roughly between $45 and $85 (again depending on where you are). That said, horns sitting for a decade often need more than that, usually because of the age of the pads (or if they have been eaten by bugs). A full repad of a tenor (sometimes a necessity in these cases) will almost definitely run north of $350 and sometimes $600 or more.
Again, an assesment by a trusted technician in person is the best way to understand what your horn needs. It should only take a few minutes for a good technician to make an accurate assesment. I hope this was helpful in some way, and good luck on the sale!