Here's a few random tips for you. I hope you find them helpful!
Always document all flaws, even if they're not things you consider flaws, and post pictures of them. Some flaws that get missed by a lot of sellers include: cancer spots, writing under the feet, head and body color mismatch, rattling, and of course smells. If you smoke, even if you never smoke near the ponies, put that in the description. And make sure to add any pets in the home, and if you clean the ponies with scented soaps or shampoos. Some people are sensitive.
Documenting all this stuff helps you in two ways. First, as long as your buyer actually reads the description and looks at the pictures(sometimes they don't...), they know what they're getting. You shouldn't have any buyers coming back to you and wanting to return a pony because it wasn't as expected, which is a huge hassle for you, and usually ends up costing you money for shipping the pony back and forth. Second, if it's all there to see already, you shouldn't have a bunch of buyers asking you for more pictures or better descriptions, which can save you A LOT of time if you've got several interested buyers and a lot of ponies for sale.
I like to be crazy picky with my descriptions so any surprises on the buyer's end are happy ones:)
I keep all bubble envelopes and boxes I get. Let your buyer know if you're using "recycled" packaging material "to save them money". Most buyers don't care as long as their item arrives ok, but sometimes you run into the odd one that does. And make sure if you are recycling boxes and envelopes, they're in good enough condition to actually protect your item.
Charging a handling fee to cover materials is ok, just be up front about it, and make sure you use the handling fee to pay for the packaging materials. If you charge them for a bubble envelope, don't send it in a recycled envelope. Buyers generally don't like to pay handling fees, and some will avoid sellers who charge them, but it is acceptable to charge handling.
Do NOT overload bubble mailers! I've had a few bubble envelopes arrive with too much stuff crammed into them, and they tend to burst at the seams when that happens. Taping all the seams of the bubble mailer with packaging tape can help prevent these types of accidents, but it's best to use the right sized envelope to begin with.
If you're sending small items, make sure they're contained in a baggie and not just loose floating around the envelope or box. I had a bubble mailer full of accessories burst, and the seller just dumped the accessories in the envelope. Several were lost, and their was a trail of pony bits leading up to my door from where the mailman carried it.
When packing up things to sell, I like to cover all addresses with clear packing tape to protect them from being torn or getting wet. I've gotten some inkjet printed packages with smeared or torn addresses because of the weather or rough handling. If the address becomes unreadable, it can't be delivered:p