I had some thought as well when I saw the posts about some 200k cards on someone's want list. Fairly early on when I came on this site, I had started to add every baseball card I did not have. And I watched my want list grow and grow. In sets that I had only a couple of cards, I now had 700 wants. It got me thinking that there should be some personal cut off to what I added to my want list. So I decided that everything to 1998 from the major brands (when Score and for a few years Donruss & Leaf stopped producing baseball cards) would be on my want list. After that, I would only add cards of sets that I already had a large number of cards from, 40-50%. Reason was that I often buy large lots from collectors quitting or downsizing so I could get portions of sets at a much cheaper cost than trading. If I had been trading for some of those cards, all I would end up with is that many more duplicates when I bought a large lot. Having said that, I have added sets to my want list when building a larger trade here to allow the other member to increase the cards he could send me and get what he wanted.
Doing so I have tried to keep my want list to around 40k. Right now it sits at 45k. I have also managed to complete sets or get them very near completion instead of having many sets of only having a few cards in each. Since I started my "Sets Near Completion" list about a year ago, where I need 5 cards or less to complete the set, I have seen it grow from 25 to almost 40 entries even though I have completed several sets from it. As for how long it may take to get all the cards on my want list, considering that I have traded for over 10k cards in 2020 alone plus what I have bought locally, the only thing not realistic about it is getting the extremely high value cards such as the 1952 Mantle.
I have also thought about "what if I ever had to move to where I had less space than the 1500 sq ft that I have now for my collection?" Well, I simply quickly put that thought out of my mind when it does happen. But I know my youngest son thinks about it.
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