I didn’t say that this particular kit was available on Ebay, just that Heavy Hobby products are available there. This kit has just been announced; I doubt it’s been released yet. If you searched on Ebay for Heavy Hobby I’m sure you found modelfactory123 selling their products. When this one is available I’m sure you will find it there.
It’s double the kit price since it is over double the kit. The kit retails for $20-25. Two Aber turned barrels would run about $15-20. Based on the latest 3D print upgrades from various manufacturers, the parts shown here would run in the $10-15 range plus another $20-25 for the tracks. And the bit of PE included (however needless it may be) is probably a $5-10 touch. So all told this amounts to $70-105. So I don’t think the price is out of line; if anything is a pretty good deal.