Personally, I don't like floppy flowers. If they aren't stiff enough, I would starch them. Maybe try starching one first and see what you think after that. : ) Good luck at your flea market!!!
Hope you sell LOTS!
ETA: If the flowers (usually daisies) I buy aren't stiff enough, I separate them out as usual, then spray them with hairspray or starch and blowdry them (or you can bake them). This has worked really well for me. I learned my lesson after one of the first daisies I made. it was semi-stiff, so I thought it would be ok, but after my dd has worn it for about 8 months now, it's floppy. I may still spray it and blowdry it even though it's already all assembled. I think it'll still work ok