Never pay cash. Caveat emptor and “if it sounds too good to be true” it probably is. Beware of any one to tries to sell you the golden path to easy riches.
Many places will not take a credit card because of the return policy on purchased items (you could return the item if the price went down and buy it again at the lower spot price) Also the 3% fee is huge when dealing with precious metals. A bankwire or check is your best bet, but that leaves a paper trail for the feds.