no clue what tanks are worth there but I wouldn't be paying anymore then 500$ for a 6x2x2 second hand in good condition. Brand new id be expecting around 800-1000.
I think resealing smaller tanks (under 220g) is quite easy pending its the seal inside the tank that needs replacing, if the outer seal is going its a real pita and can be costly. just cut the old inner seal of with sharp razors, the fresher the razor the better it will remove the silicone, you also must insure the slimy looking stuff is removed with razors or scrubbing.
Then I would clean all the areas with methylated spirits and dry it off with paper towels ( try to avoid getting to much on the outer silicone seal ), add some duct tape around the seams to keep them clean. Apply the silicone then you run your finger over it or the end of a pipe etc adding a bit of pressure as you want to push it right up against the outer seal but no so much you push it out the sides of finger, I think this is the hardest part and a bit hard to explain, If you want it to look clean id suggest practicing on a smaller tank or something.
Hope this helps.