If you stick with reloading for the long term, I very much doubt you'll be "unloading" your single stage on Gunbroker or Ebay. Even when the time comes for a progressive, your single stage will remain very useful, and you will probably keep it and use it often. I'm starting to look at progressives myself, but after loading for a year on the single stage (Hornady LNL Classic), I have come to understand what others meant when they gave me this same advice. My single stage is here to stay, regardless of how much further I dive into this hobby.
I bet a lot of the most experienced reloaders on this board would agree with this analysis: If you buy a single stage first, you may find that you don't need a progressive for a long time. If you buy a progressive first, you will very likely find the need for a single stage in the near future.