I've collected for years and have sold off huge chunks of my collections periodically. To me, that's honestly the best part of collecting... Getting a pony, enjoying it, then passing it along to someone else who will no doubt be very happy.
(not to mention saving room in my house!) One of my favorite things is when I sell someone a pony they REALLY want, and they show me pictures of the pony once it's arrived to them. It makes me happy to know they're happy.
I've recently sold the bulk of my G1 collection again and vowed to keep it under 100 ponies! (Hey, can't blame a guy for trying right? Yes, I have a pony problem...) To decide which ones I was going to keep was kind of easy really. I connected the most with the characters in the original g1 movies and cartoons as a kid, so just keeping it to those ponies has been really fun. I love hunting them down on ebay and giving them the scrub & conditioner treatment as soon as they arrive. No matter how clean they come to me, it's always sort of a fun ritual for me to give them an immediate spa treatment before joining their friends on the shelf.