I collect both memorabilia and cards. I haven't sold a lot, but when I have it is because I am not getting the enjoyment I thought I would from the item(s), or the items are taking space I would rather use for something else (and selling these items will help pay for the new item).

I usually look to sell items that have some combination of (1) little to no sentimental attachment, (2) replaceable without too much effort, and (3) duplicative or close to duplicative of things in my collection. As long as (1) and (2) apply, it hasn't been too difficult to bring myself to sell.