If in any doubt, never by shy to ask/tell (politely, please) how you want a poster shipped.
They sometimes take no notice, and I know of no professional dealer who will object to this,
simply because at some time all dealers have received shoddily packed goods.
Folding a one panel again is absurd. Sounds more like the seller was pissed off that his poster sold for 99 cents.
Oddly, if one starts a sale at 99 cents, one should expect it and be happy to wear it.
Phil