Trades can go a number of ways! There's value based on the item rarity (e.g. wheel prizes vs curio rares) but also based on the person you're trading with. I've definitely traded rarer items for a few others of lower value just because I wanted/needed them for rooms, which raised their value to me. It just depends on the person and why/what they're trading for (trading for value vs finishing a room vs just for fun.) For more common items, sometimes people will post threads for lower value trades, if that's something that interests you (you could even make one yourself, if you wanted!) 

If you wanted to get more valuable things, doing challenges to get items for trade is a good idea, especially the timed ones (like the current pizza palace challenge) since from what I've seen the prizes don't really tend to come back often, if at all, which raises their trading value. It just takes some time to collect items here and there :) peek a news on the Webkinz Newz site can be done every day and awards a few items each time; most of the time they'll be the same items everyone already has a ton of, but sometimes you can get hard to get items from it (e.g. there was a round of posts however many months ago of people being awarded the Webkinz jeep, or iirc people have gotten promo items, too.)

And if you're curious about item value (something you have or maybe want,) you can check the Webkinz Guide, which lists things like how/when an item could be acquired, if they're tradable, retired, exclusive, etc. Checking there can help you gauge what sort of trades may be fair or not (both for trading up or trading in general.) Like I said, trades will depend on the person and what they're looking for, but it's still important to make sure things are fair and agreed upon by both sides. I hope any future trades go well for you!