What lies dead one hundred feet in the air?

I’m an arrow, but I never fly, Tho lift me up, and posts climb high. I’m worth no coin, yet you’ll agree, Nobody wants the minus from me.

The first time you meet me, you can blame another, but the second time you should look at yourself. Kings once kept me by their side, but now I live amongst you believing myself wise.

What word has five letters but is pronounced as only one letter?

I am a poison, but can be bought at the store. I can help a conversation, or make you speak no more. Many have been lost to me, and locked behind doors. When I am done with you, you might end up on all fours.