I can buy this for 900 dollars but I am not sure if it is a good buy, the marker on the knife looks a bit strange to me

That's a pretty rough looking maker mark which I'm inclined to say is questionable. How about some pics of the rest of the hilt & blade motto?

alcoso is one mark how have not the triangle RADJ
but this mark i don't love it
poor gravure for me
karl
see other pics please

The mark looks rough indeed, but it's hard to judge the dagger basing on one detail only. RAD hewers unlike SA/SS daggers are not easy to put together from different parts, so if the rest of the item looks correct, it's a good sign.