I do not own a Marocchi / and to my knowledge, I've never seen one of their guns in person. I remember seeing reviews a few years ago on Marocchi O/U's in sporting clays magazines - but I had not heard or seen that they were making any single barrel Trap guns.
Marocchi is an Italian mfg / and there were reports awhile back that they were making guns for Daly, etc - and I think its fair to say, Marocchi is a middel of the road mfg. I think several years ago, their O/U's were selling for around $ 2,500 - $3,000. I suspect their single barrel Trap guns were in the same price range / and you said you have 2 stocks which is a little unusual. So I guess in reasonable condition - the gun might very well be worth $ 2,000 / maybe more - but without a picture, I don't know that I can get any more specific.
A S&W 686 - in my area - is probably worth $ 500 - $ 750 depending on the specific model, barrel length, etc.
So on the surface - it looks like a good trade / if you want a single barrel Trap gun...
I see a lot of single barrel Trap guns around / Browning, Beretta, Perazzi, Krieghoff, Ljutic, etc - but I have not seen a Marocchi - and depending on the fit, finish, feel, weight .... its really hard to tell what the gun is worth in terms of value. It may be well known in some Trap shooter circles / I'm not a big Trap shooter ....and while I own 2 single barrel Trap guns - they are both Browning's ( one is a BT-99 and one is a BT-100 )... and a gun that is not well known may not retain its value...if that matters to you.