From everything I have read, it takes a while to get to a point where you have stuff you want to sell for a profit unless you build it into your strategy early on. The market also fluctuates and takes time to go in either direction. A lot of the long term collectors seem to have a good supply and are continually selling or flipping fairly consistently as they know all the cards and market better (i.e. the most market knowledge and foresight). The problem for me is, I tend to focus on fairly high end cards and I try to get them at the best prices that I can usually by “sniping” auctions or just consistently searching around for deals from the market knowledge I have so far (also a lot of patience). Almost all of the cards I have bought I feel I could make a profit on, but the issue I face is like yourself I want to keep them for my collection long term mainly.
Basically I am yet to almost never buy a 2nd or 3rd of the same card because A) I rarely find it at the same or lower price due to rarity/opportunity cost of the 1st card I acquired B) It’s money that I’d sooner put towards other card(s) instead. So until I reach my longer term collecting goals, it’s going to be a while before I have the option to sell more unless I somehow free up more capital to spend and obtain multiple copies of a card I’d like to flip or sell.
The good thing for me personally is I have a steady cashflow from a secondary income I established long ago which I put into collectibles but also other assets so I never really feel a ‘need’ to sell any cards at this point even if I had an excess supply to do so. Maybe at some point down the road I’ll eventually have cards I can sell off, but personally as I build my collection it hasn’t been put into my strategy yet.
You may want to check out these threads/articles, they may be really helpful for you:
www.elitefourum.com/t/investing-in-pokemon-and-financial-responsibility/28344/1
www.elitefourum.com/t/financially-smart-collecting-vs-investing-in-pokemon/25996/1