I read this thread yesterday and tought about it for a while. I think some have said much of the why but I will add my view on it as well. The majority of the cards from 1985 to 2021 can be had for less than a penny per card when I buy larger lots of which there are enough available even in my rural area. These large lots have commons, semi-stars, some stars and even HoF in them. They also have inserts of the lesser players. As the cards get older in the lots fr slae the price goes up. Last year I bought a shoe box full of 1952-1962 cards and paid about $1 per card. NO they were not mint and no there were not a lot of stars in the box. But it did help fill some of my older sets.
So even if I did open the doors wide for trading modern for vintage, is anyone going to offer me 100 2012 or 2014 commons for 1 1959 common card? Haven't seen any offers like that yet. More often, the other side thinks 3 commons from 1995 is sufficient for that vintage card they so preciously want but aren't willing to spend $2 or $3 for.
The other reason has been mentioned - I hope another vintage card collector who has also accumulated a large # of traders in the vintage years, comes along and we can trade vintage for vintage. Basically, if you don't have the cards to attract these traders, then you will have to go out and buy them. I did and I don't wish to deplete my vintage traders on modern cards that I can find easily.
Bruno
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Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0