Strange timing here, but I took my 6" blue 22 Diamondback out last night to make sure it had a fresh coat of protective oil. The Diamondback takes carbon steel and shapes it into elegant Beauty. I snapped a cell phone picture before it was re-oiled.
Most people end up regretting the trades they make. This is especially true when the item being traded is something that is hard to replace.
The trade is not a slam dunk, super deal in your favor. It is just a market value trade. The Colt is rising faster in value than the other two pistols. I disagree that any of these is currently declining in value, especially if we are talking long term.
So unless you are aching in your loins for the two firearms being offered, I would keep the Diamondback and try buying the Beretta outright from this person. If you do not have the cash for an outright purchase see if the seller would agree to a Lay-A-Way deal. 25% down, 25% in 30, 25% in 60 and paid off in 90 days. Most of the fine firearms I purchased when I was young and starting out were done thanks to Lay-A-Way. I understand the "I want it Now" feeling, that feeling is why people make trades they regret. Resist that feeling.
Building a collection that your friends and family will love to look at and talk about with you is nothing more than resisting the urge to trade off what you already own. Do this for 10-20-30 years and you will surprise even yourself at how broad and encompassing your collection has become.