You'd need to contact the suppliers and see how much you can get each unit for and whether there is a discount for multiple buys ? Then work out the customs and p&p charges from the US, then get a price for sending them to the members once they've landed over here.
You should end up with a price with which you can guarantee each hammock to be delivered to the members who participate, post that price up on your GB thread and then see how much interest it generates. Presumably you'll need a minimum number to make the GB happen.
Taking payments yourself and arranging the postage etc can be a headache when folk start dropping out/blanking your PMs/or just generally giving back word.
Whether you decide to make the bulk purchase yourself and then recover costs from the members is up to you, personally I'd get payment in first before you make the order with HH.