I've used pax, but not from ebay. Seemed pretty ok, if you dont mind the metal herb chamber/vapor path and limited, not self regulating temperature range, better than atmos or iolite for example. As to skyda and ago, they are just rebranded direct drip ecigs, you can get a variable voltage model not rebranded for $15. The ago type works fine on herb, but just burns wax. Honestly, all the pen type vapes are the same, rebranded ecigs that use a nichrome coil to either heat air which vapes herb, or if they are branded as wax vapes, they just burn the wax with the nichrome coil. The pen type/ecig vapes are literally alll made in the same chinese factories, regardless of brand or price they are. Ebay vapes I find to be a bit sketchy, at least the tabletop ones. I've gotten a few ceramic heating element tabletop ones from ebay that had some grease on the heating element, which made a huge quantity of foul smoke upon the first several heat ups. But those are manufactured by different folks from the portable vapes, so you likely wont have that shitty an experience.