Guy has one for sale locally,he says it shoot fine but the finish leaves something to be desired.
Serial number is 57xxx and numbers match on slide and frame and is made in Belgium.

He says it is from around 1950.

I know NOTHING about this make of pistol,so any help would be appreciated.

I'm not an expert, by any means, but the 57 at the beginning of the Serial Number should indicate it was manufactured in 1957.  The cut out on the side of the slide was stopped in 1958, according to my book of Gun Values.  So it looks like a last production before they went to the smooth slide.  The grips look to be original, and the picture is not good enough to get a really good feel for what the finish is like.  

The question is how much does he want for it, and does he have extra mags or holsters with it?

Dating by FN factory SN alone can be "complicated" with BHP pistols...particularly those produced in the '50s, as FN often used "Contract Numbers" instead of sequential commercial serial numbers. Based upon the small pics though, your 57XXX range BHP appears to be commercially serial numbered, and as such would date to '53/'54. FWIW, because of FN's production methods, a thorough inspection of the parts and their coded markings is required to get it any closer.

Hope that helps

EDIT TO ADD:

WADR, the 57 in the 57XXX is part of the SN and not a SN prefix as found in the much later "C" series of FN BHP pistols...they began in "69" and ran through "77" as in 69CXXXXX would be 1969 and 77CXXXXX would be 1977...

Asking $400,comes with one mag.

Looks like a MK-I based on the internal extractor.
Those parts are becoming more difficult to find as the external extractors have been in use for quite some time.
The older guns like that also have the humped feedramp that can be problematic with anything but ball ammo.
It's also got the smaller thumb safety and sights which many find less desirable.

Personally, for $400 I'd be looking into the Izzy surplus MK-III's that are floating around right now.
Finish looks pretty beat as well.

$300 would be my limit.

Z nailed it pretty well.  No collector value going on there and has a few challenges for a shooter.  Might be better off in that price range with one of the surplus guns hitting the market.

FWIW and WADR, on the technical side :

OP's 57XXX pistol is a "classic" FN BHP, not a Canadian Inglis. FN did not designate any of their BHP pistols as "MkI" (in spite of Wicki ), Inglis on the other hand produced the various wartime MkI pistols. FN's first "Mk" designation came with their MkII, a specific updated model designed to compete in the Military/LE market. FN MkIIs were produced between late1981 and 1989, and they were produced concurrent with regular production BHPs as well as the early MkIIIs. The MkII introduced improved sights, the current hollowpoint friendly feedramp, current ambi-safety, current Firing Pin Safety and current plastic contour grips...which were slowly incorporated into regular production as well as the later MkIII and MkIII based pistols.

Offer him $350 cash if it is a face to face deal. Any early High Power that is in good mechanical shape is worth that, as long as there is no pitting damage to the finish. I paid $350 shipped to my door for this '47 High Power a few months ago.

Thanks for the replies,not knowing anything about them,I really thought something that old and in operable condition would be worth more.

To some they are.

I'd offer $400 for that gun, IF the barrel was in good shape and it didn't show excessive wear internally.

But then again, I'm a sucker for an old BHP

$3 beans or pass for something a bit newer like the surplus Izzy's

This.

No such animal as a "MK-I" FN/Browniing Hi Power.

Pre "MkII" Hi Powers were not referred to as Mk I's, but simply the "Standard Model" (encompassing changes in 1962, 1965 & 1973)

The OP's Hi Power would easily go $500+ on Gunbroker.

The Inglis was designated the MkI Hi-Power in British Commonwealth forces.  I imagine that is why Browning designated their updated Hi-Power they submitted for various military contracts as the MkII.