First know your ponies! Dream Valley and the The My Little Pony Collector Website both have simple, wonderful, and easy to use sections that will help you identify your ponies.
There is nothing worse than traders remourse. Don't accidently trade off that pony or accessory without knowing exactly what a fair trade for it would be. Again, ask, ask, ask! Help is always just an email or a post away.
Be aware that Euro ponies were released with different names than their US counterparts, and many US ponies were released with the same names. For example there is a Euro flower pony Daffodil and a mail order birthflower Daffodil, three newborn twins named Sniffles, two named Snookums and two sets of Rattles and Tattles. I mention this because when negotiating a trade, buy or sale over the net where no picture has changed hands, always, ALWAYS, try to include a desciption of the pony with symbol color and year and set of the pony in question. It saves a lot of confusion and time. Make sure when writing up your want and trade lists to also include this info, ex: 'yr 5 Newborn Sniffles, pink/mittens'.
Reputable traders, just as reputable sellers, will always welcome inquires concerning references. Ask the trader to supply you with a list of references, if not listed on their site, ask other collectors. There are even quite a few reference sites on the net:
Plus other collectors are more than willing to share their experiences, so again, ask ask, ask! Once you have agreed upon a trade, it is time to package and send out your item. (Please see How do I Ship MLP's?)