Sauer guns are generally regarded as well made, as are AYA. Sauers are German, AYA is an old Spanish firm. I didn't know they ever collaborated. I'd roughly guess your guns value in the $1000 range. BUT that can vary a lot depending on details. Prospective buyers will want to know barrel length, chokes, and stock dimensions - LOP and drop at point of comb and heel. recoil pad? One full length picure will help too.
Never heard of Marshwood, pics may help.
I'm actually kinda interested in the Sauer/AYA, please send PM if you'd like to discuss.