Sending out Offers in marketplace.

It was just a question on whether I have to wait for the first offer to expire or decline to make another one. 

Yes, that's the way to do it. Wait for your first offer to expire before making another offer for another item. Or, you can ask the seller of the first offer nicely to reject your offer, so that you can safely place the offer on the other item. I have done this before. It's of course on the seller's discretion, but, if you ask politely, they will do it.

Please don’t assume or make things up. I DID NOT back out of an accepted offer. 
It was just a question on whether I have to wait for the first offer to expire or decline to make another one. 

The AG people will access you a fine if the offer is accepted and you back out.

A formal withdrawl may help your case.

At a minimum you risk the seller leaving you negative feedback if he accepts your offer within the specified time limit and you decide not to go through with the transaction.

Yeah. I just want to know if it’s like the eBay system where after you send offer, and the seller accept, it’s your responsibility to pay for the item. 
Sounds like AG has no such system. 

Send a message to the seller telling him the offer is withdrawn.

you could tell the other seller(s) that you made someone else an offer, no response, waiting for a reply within ___ hrs, and if declined, you would like to offer them ____.

IOW, get your foot halfway in the door.