Beckett is a guide, no one should take it for the gospel of values or what constitute a fair trade. If the other trader wants something more of value and you balk you don't have to trade, but don't belittle them for being up front with what they want. These two excerpts are taken directly from my site:
I'm a set builder. My close to complete sets are listed on this site, they are my first priority. I have no problem trading your needs for my needs on a BV for BV basis. Sometimes I will find another set builder where one of us can make significant progress if the other would pull from their partial set. In acknowledging the sacrifice, I am the first to suggest that the person be properly compensated with additional value in the trade. I am also willing to have the set builder types help me with my not so close to complete sets if I have a lot available to trade with them. I just have a tough time when the player collector wants me to trade that tough card they need for like 6 of the 40 commons I need in a low priority set that I'm building.
And this:
I've set up a trade page, and I have tons more that I can trade at even BV, if you have a key card that finishes a set for me you might be entitled to a bonus, I'm open to offers. I may be willing to trade my parallels in my personal collection, but it HAS TO BE for something on my "close to finished sets" wantlist, and probably a parallel at that, to trade at close to BV for BV, otherwise I will take other of your unwanted parallels and SPs from certain sets, generally on a 3 to 1 basis, although larger trades and lessor players demanded such as for set or team collectors will be given greater latitude as well as if ultimately I have a better card in the end it could be down to 2 to 1. If you want a $6 parallel of the player you collect you may just have to trade six $3 common anybodys to get it, but if you have a $10 one to go with a $3 one that is probably going to get it done. While some may find this to be unfair, they must understand that I don't collect by player so a BV for BV that doesn't fill a specific need makes no sense and I'd just as soon be mailing myself my own cards as these parallels are just fungible items to me. Just as player collectors would balk at trading one of their non-duplicates of that player, I prefer not to trade my parallels, but I give an option and that is to trade in my favor. It's no different from many people's policies regarding GU for GU, or Autographs for Autographs, or that they won't trade a big card for a bunch of little ones. I do provide a service of having a great inventory to pull from, and I'm not choosy as I do like many full tough parallels.