My uess would be that it is only a monopoly if there are no other sellers in the background - e.g., if you are the only producer or only retailer in the world/country/town, then you have a monololy. The situation you describe could mean that there is no other person selliong [i]at that moment[/] but it may bethen that all the seller has to do is to wait until another seller appears (e.g., the shop next door opens after having been shut for a period.) This would, I spose, be a temproary monopoly, but it only works if the buyers can't wait for whatever period the other seller(s) remain out of the market.
My knowledge of economics is slight, so I may be wrong here, but I loive in the certainty that, if this is so, someone will tell me so in fairly short order
duopsony
