Primer is your friend.  I use Zinser brand Bullseye 1.2.3. Primer For All Surfaces and it really helps.  I've painted ponies without it before and sometimes the paint doesn't want to stick in spots (which is super annoying).  But I haven't had any issues with a thin layer of primer in place first. 

If you want a smooth look don't be afraid to use fine sandpaper over what you've already painted and then do another coat of paint.  It can really help.  Also if your pony has rough spots in the plastic you can do a bit of sanding before you start painting to help get a smooth base.

The sky's the limit.  Remember that there are no mistakes, only Artistic Choices! 

Wow!  Your customs are gorgeous!  I love the 3D additions on them.  It makes them so much fun!

As for how much people would pay, I don't have any idea.  You could always put one on Ebay and see what people are willing to pay for them and go from there if no one on here has any suggestions.  Just decide on the minimum amount you'll accept for your work and go up from there. 

Your work is beautiful. 

Thanks so much for all your comments 

I'll post a link on the for sale forum when I put another batch of customs/artwork on Ebay. 

Until then, be sure to check out my DeviantArt page and see the cutie mark trinket boxes and pony piggie banks I painted. 

30 really isn't that old, unless you have some underlying health issue that will shorten your lifespan.  If it makes you feel better, I'm older than you are and don't have kids either.  Even if you don't have children now, maybe one day you will.  Maybe you'll adopt.  Or maybe you'll marry someone who has their own children.  Maybe you'll get a niece or nephew that shares your love of ponies.  Personally, I think it's a bit early to be worried about where your ponies are going to end up.  If it's really bugging you, though, maybe you should make a record of which ponies you have and what they're worth or maybe you could pick someplace for them to go (a charity or a friend) and make sure that someone close to you knows that's where they're supposed to end up if anything happens to you.

I started collecting ponies when I was 5.  My very first was Baby Glory, so you can figure out how old I am from that.  Now I not only collect, but I do custom work as well, which my brother sells for me on Ebay.  My favorite color is purple.  I work at a library.  I'm fluent in sarcasm.  I dabble in writing.  And I'm waiting for my Prince Charming to get his act together and show up.