Fish Tank Sealant

I bought a second hand tank and rubber has rotted a bit up and leaks if the tank is filled, I saw some sealant on eBay, it will be allowed only to put it on the missing or should i replace the whole band

Always make a band ... Silicone does not adhere well on the old silicone.
If you have silicone in a degraded area, there may be others.So inspect every seam by running a finger to dry up the search for "pens" (free edges) and silicon dry "fire" from the window ... You can find other areas that need attention.
This is important because the silicone is what holds all this weight of water and ... not funny to have a pop loose panel to 4:00 am.

I would always replace the whole band!

I was in the same position. I removed all the tape and replaced with new sealant. It worked very well. Just be sure and follow the instructions.

Some sealant on ebay is the broad definition ... it may work, it can not, it can cause your goldfish brutally attack and eat your neighbor's cat. Be careful, or better yet give us a better description of "some sealant on Ebay"
Edit: This is only if the ad says ... ;-). However, if I were you I would go to local fish store reputable and ask them if they sell any silicone sealant. Not all sealers are equal and all marketed as "The best on the market. Some silicones contain chemicals that inhibit the growth of mold. They could be excellent kitchen or bathroom and completely wrong for the fish tank.
For Parachromis: OK, arises at 4:00 fistank panel is not funny, when it can be considered fun???