Thanks for the replies so far. I guess I just got worn down on this trade and caved in on Friday to finish it out. It's been going on a long time, but I wasn't trying to be a jerk... just had some differences on values.
The main issue is that I use the Standard Catalog for getting values for modern, Post WWII vintage, and Pre-war vintage cards. So many other traders use monthly Beckett's for everything though. I figured out a long time ago that paying $10/month for Beckett (I guess it's cheaper if you have a subscription though) is not worth it since the bulk of the prices listed are for 1990's and 2000's sets which I don't have much interest in. They cut out more than half the players/cards from the other years so they can put all those insert and parallel sets in there.

I think the yearly Std Catalog is plenty accurate for my needs. Unfortunately, Beckett has some values that are...let's say... overly optimistic? For example, Beckett had $6 for the '81 Donruss Rickey Henderson whereas the Std Catalog I had showed it at $2.
Anyways... I'm OK with the trade since it helps me complete a set that I'm working on. I guess in the future though I'm going to have to be tougher and just state no vintage-for-modern trades. The valuation always seems to be in question and I usually end up trading away vintage because I'm one of the few folks that still wants to build (rather than outright buy) all those '80s sets that I knew as a kid.
Thanks again for the advice!