You've got some great advice here. As someone who's been forced several times to sell my collection when times are tough, I sympathize. Ponies can be bought back, but some, as stated already, will be near to impossible to find again. Be careful when you're going through your collection. Take your time if you can. Pay attention to ponies that are special - the impossibly rare ones and ones that have sentimental value. The ones with special circumstances, a special gift, or a pony that you really really love.
Sure they are just things, but the feelings that come with them have no value. You don't want any regrets, believe me. <3