There isn't really a guide per-se, but here are some things you should be aware of when trading.
1. SP cards are generally more valuable than normal villager cards. Just due to the math of only ever getting one SP per 6-card pack, they're generally more valuable. Usually, people will only trade SP cards for other SP cards, although exceptions are always possible. Generally, in those cases, people will trade 2 villagers for 1 SP, but it absolutely varies from trade to trade.
2. Series 5 Villager cards are a bit less valuable than other villagers. Just due to how the Series 5 cards were produced (the ratio of Villagers to SP cards in that series is a bit skewed), people generally end up with a ton of duplicate Series 5 villagers before they finish the set. Not saying you'll never get rid of them, but most active traders will already have a full set of those villagers by now.
3. The vast majority of Series 1-4 villagers are worth exactly the same to traders, but there are a couple of exceptions. There's really only a very small handful of villager cards that are actually more valuable than others, and those are the villagers that are just more popular in the games. Cards like Marshal, Tangy, Merengue, Ankha, etc. And even then, ever since Target started getting reliable restocks of the Series 1-4 cards, even those cards aren't as valuable as they were even a few months ago. It might be SLIGHTLY more difficult to find specific cards like that, but you can absolutely trade for them on the forums without having to throw a dozen cards at it, and likewise, trading one of those "valuable" cards for a more "common" one is no big loss.