You can only purchase them before the merger - after the merger, they act like a stock option that you can either execute and pay the $12 (to the new company) to convert to a share (plus any taxes due) or do nothing and lose your initial purchase price for the warrant. Before the merger, you can trade them on the open market (buy and or sell to other investors) just like they were stocks.

No one NEEDS to sell them pre merger but you do not have that option available to you post merger so ... if you think the stock will drop in price shortly after merger (which I do) then it may make since to sell the warrants pre merger when they are trading at a premium, wait for the merger to happen, buy shares of stock after the drop, avoid paying the $12 fee to convert the warrant and any associated taxes you may owe.

You can only purchase them before the merger - after the merger, they act like a stock option that you can either execute and pay the $12 to convert to a share (plus any taxes due) or do nothing and lose your initial purchase price for the warrant. Before the merger, you can buy or sell them on the open market just like they were stocks.