Many sellers makes for many rules....Some may want all the fps on a single GB, others may want them spread around. A good seller will allow you to grab a 1st, 2nd or 3rd place on a GB, thus decreasing your costs with an fps payback, but that doesn't happen often.
Bottom line is...In this game, like life, it's buyer beware....there are crooks out there....do your homework, ask around, make sure the seller has a good track record, because INNO doesn't care if you make a poor decision.
The normal process after the payment is made is for the buyer to post up trades at 2:1 using their age goods for the goods needed for the GB. It's important that the buyer makes the trades, because if the seller makes them they would be gone before the buyer could take them, ie
Prices also vary between worlds and between seller's. Generally new worlds have higher prices for upper age goods, since fewer players have reached those ages. More established worlds have lower prices since there are more players selling those goods. I have also seen quotes vary as much as 200 FPs between the highest and lowest seller, so it pays to shop around.
The only trades I've done are 1 BA good for 2 PE / PME / CE / FE goods. Most goods sellers accept that. A few, if they're in a GvG guild may request IA goods so they can donate them to the treasury without having to trade up.
Buyer beware is right. In one of my cities, when I was getting quotes for CF and Traz goods, I got a quote from a goods seller who was always advertising on Global Chat. His quote was about 175 FPs higher than the one other quote I had already received. Next thing I know, I'm on his Group Message about which GB he wanted all his buyers to add to. Every few hours he'd shift everyone over to another GB, then another one, and another one, so none of his goods buyers could ever accumulate enough points on a single level of a single GB to get a reward spot. He was selling those at 1.85% to the Arc 80 people. He was using his goods buyers to provide the lock in FPs on his GBs. He just kept moving them from one GB to the next to prepping them for the next Arc 80 donation.
He had about 25-30 customers dropping FPs onto his GBs. He had the worst possible deal all the way around, taking advantage of the players too eager to do their homework and shop around. When I tried calling him out on his practices, his response was, "I'm not taking advantage of anyone. I give them a quote and sign them up, just like I did you. They add their FPs and when they're paid, I trade them the goods. I'm happy, they're happy."
Needless to say, I went another way. I had sent messages to the higher ranked players on my friends tab asking for recommendations of established goods sellers with good reputations. One of them responded that they had recently made it to PE and was just starting to sell PE goods. Since we had done a FP swap on his CF when I was chasing prints, I trusted him. He ended up giving me the best price, over 200 FPs lower than Yo-Yo's bid, I could pay over time, and my FPs onto any of his GBs. I was easily able to get first on multiple GBs and multiple levels the FP rewards reducing my cost further. I got such a good deal, once I got done with CF goods, I kept paying until I had a Traz set also.
In another world, I shopped around, got the going rate, then messaged my guild mates in PE asking if anyone wanted to trade PE goods for FPs. My guild founder said he would, I told him what the going rate was, and he said he'd do it for half. Again, I got CF goods and just kept paying until I'd gotten a Traz set as well.
Compared to the players I chat with, I've paid over a thousand FPs less for upper age goods than they have. They were always in a rush to build, the minute that last print came in. Sure, they got their GBs up a bit quicker, but I was able to level up a bit faster with the savings.
I also don't see how putting them on a building where 500 will get them squat is less greedy than putting them on a building where i have to ask for 600 so i feel comfortable putting them on a spot I could've given away for 300.