I don't buy to speculate, but I do sell items as my collecting tastes change. In most cases where I bought an item on the resale market, I rarely remember what I paid for it so have no idea if I made a "profit" or a "loss" when I sell it and don't care enough to keep track. If I bought the item new from a dealer, I can look up on the internet the price I paid and add in taxes and delivery, if any, to come up with my cost. I've sold items for less than I paid and for more than I paid - overall it seems to work out about even.
To me the only real speculation is when someone buys up several of a hard to find item to hold for years in the hopes prices will go up and then sell for a profit and deprive real collectors of the chance to get one when it is issued. Given the state of the TS industry with oversupply and lots of choice for models, I think speculation is extremely risky and not worth the effort. I doubt if speculators are doubling or tripling their initial cost on sale. Just look at popular items that sold out 3 or 4 years ago. How many of them are now selling for big gains? Not many. Anyone disagrees - name them.
As for people who buy "one" for their collection and later decide to sell it, since no one compensates them if it sells for less than they paid, I don't think much of people who criticize if the collector gets more on resale than he paid originally. IMO, this speculation fearmongering is way overdone and does not have much impact on the industry or on real collectors. Remember the panic to buy a Tilley? How hard is it to get one now? How expensive?
Terry