I think prices are very subjective and vary widely from different locations of the country/world, You do not indicate if your B6000 is 2wd or 4wd which is an important factor to many potential purchasers, Just as an example I purchased a late 70's B5100E a few months ago, The tractor runs great and needed only routine maintenance and no major repairs, I paid $1400 delivered to my door, Your B6000 is very similar in horsepower ratings,capabilities,size and appearance so that may give you some where to start from. Some would say I paid too much while others would say I got a good deal, The thing is I paid what I considered a fair price for what I was getting in return. I use this same logic when selling an item, I will not give it away for less than I have invested(minus age/use depreciation) but I will not price it so high that I discourage potential buyers, I do generally set my starting price somewhat higher than I am willing to settle for as it is pretty much a given that there will be some price haggling by any potential buyer and I usually indicate that I am receptive to a reasonable offer when setting my advertised price.
So basically your B6000 is worth what someone is willing to pay for it and you are willing to settle for as a sale price.
You can do some research by viewing open and closed listings on eBay,Looking for similar tractors on Craigslist and watching to see if the listing is posted for a long time and if the poster drops their price as the item is reposted, There are also several online tractor sales outlets where you can look for similar models and evaluate their pricing.
You can list it on eBay with an attractive starting price but set a reserve which is hidden from potential buyers, That way you can see what kind of interest is generated in your tractor and yet will not be forced to sell for less than you are willing to accept.
Good luck and let us know how you make out with the sale of your B6000 and the acquisition of a B7100HST
If you include the tiller with the tractor will you get more than if you sell them seperately, I have found that many times package deals work out better for the buyer than the seller, Will you need the tiller for the B7100HST you intend to purchase, Sometimes (not always) selling items seperately may generate a quicker sale and a higher sale price than a combined sale, This means that you could sell the tractor and keep the tiller or sell the tiller seperately, A buyer may or may not want the tiller so any potential buyer could be told a price for the tractor and offered the tiller for an additional price.