As others have said, unless they have sentimental value, there are very few ponies which can not be bought again over time if circumstances change.
And yes I have totally done this. When I was a shiny new member, I sold 2 Sweetie Baby Barrettes on the Arena for $20 each...a few years later wanted them back and at which time I had to pay real value which was staggeringly higher than what I had sold them for
That is one of my biggest sales regrets.