ask for close up pictures of the item you wish to inspect if its scratched it could be alright but if it is worn dont go for it (in this sport you need everything to be as new and reliable as possible)
downhilling requires a great deal of heavy duty equipment for forks i would recomend triple clamp or upside down forks with a thru axel. i do not recomend quick release it could be dangerous
i would recomend dual suspension frames but an extremely sturdy hard tail will do but it could limit your bikes ability to cope withe the harsh conditions
pedals with sharp studs are recomended not easy on the shins but you can wear shin pads to stop that. it is good to not have a front derailer and to have a bash guard and chain guides to stop damage to the chain and crank
i recomend a full face helmet with goggles, body armour shin pads and enclosed shoes with a thick sole.