I've seen fixies with mtb handlebars, sawed-off mtb handlebars, drop bars, and bullhorns. The fixie I built has drop bars, but I'm wondering if I should change that if I want to sell it.
Handlebar type and shape is largely a matter of personal preference - especially on trendy fixed-gear and single speed bikes.
I would sell it as is. If you change the handlebar there is a likely possibility that you will also have to change the brake handle. That's a lot of needless expense. Let the buyer swap it out if they want something different.
The handlebar on your fixie should be the one you want to ride. All the styles have purpose, even if it's just that it looks cool. If you're selling it, let it be. The new owner will have their own ideas about what works for them, and may just change it back.
It also requires changing more than just the bar, and can get quite pricey.
That bike looks great as it is. Keep as is, if you do have a flat bar kicking around in the garage it will not do any harm to mention that you also have that for sale e.g. for a tenner and are prepared to swap the bars over for free.
Fixies are supposed to be like single-speed track bikes and the drop-bar is part and parcel of the look if it is to be kept 'pure'. You do have to think of your market, you will be looking for a customer wanting the 'track-bike inspired faux messenger' look above a practical means of transport.
You might want to keep the seat for your next rig. If you are to change anything see if you can pick up a red seat out of your LBS workshop's bin of spare parts or get a red seat cover for the seat that is on there. That will complete the look.