It has become very simple to look up card values, and most players respect the mid price. It’s possible to swap multiple good cards for one great one; traders are flexible (especially when they have huge collections). If you have the time, in-person trading can be a great way to get value out of cards you have. For maximum consolidation, attend a regional Grand Prix and find someone working as a floor trader.
There are also good ways to trade online and deliver cards by mail. You may have to establish trust, which can be done via Facebook or some other network. I remember back in the nineties, people on AOL would compare the number of ‘refs’ or ‘recs’ from others they had traded with, and whomever had fewer would mail their cards first. These days there are scams to look out for when dealing with high-value cards - including counterfeits, which have become much more realistic.
PucaTrade is a freemium service that facilitates trades in exchange for points. I can send off a card for Puca points, and then offer my points for other cards I want. With luck, those will then come to me. You can also purchase Puca points. Puca points are an unregulated currency, and the value you get out may not be what you put in.
PucaTrade has experienced a dip in consumer confidence that resulted from awards of Puca points that didn’t correspond to cards entering the ecosystem (they were promoting the service following the launch of their new interface). At this time it may be challenging to trade for high-value cards there (although you can up your offers until they land). My expectation is that the service they provide will continue to be valuable, and over time the value of Puca points will stabilize. An alternative network, CardSphere, has launched with stronger safeguards against inflation. While it seems functional for high-value trades, it lacks the massive inventory of cheaper, older cards useful for casual formats.
Most stores that sell single cards will offer store credit (or even a little cash) for rares that you trade in. Local stores will have a buylist of cards they are interested in, usually cards that are in high demand. Larger stores may be more flexible. The largest stores I know for singles are Star City Games, Channel Fireball, Card Kingdom, and the aggregator TCGPlayer. These web sites also have a great deal of content, both articles and videos, that can help you learn all sorts of details about your favorite formats.