Buying Great Bulding kits means getting the goods you need (once you have all the BPs) from someone else because you can't produce them on your own. The common situation is being several (or many) ages away from the GB in question, like lower age players wishing to boost their game through having a Chateau or an Alcatraz.
Why is this done? Construction of a Great Building requires goods from the building's age. Typically a player can only gather goods of their age or below, and trade ratios mean trading up one age at a time would be very costly. As a result, it's not easy for a player to gather the necessary goods to construct higher age Great Buildings on their own.
Players have worked around this by finding other players willing to 'sell' the necessary goods in exchange for forge points. Because there is no formal mechanism to make such transactions, these deals are virtual 'handshake agreements' and require trust on both sides.
How it's done
The 'buyer' (the player wishing to construct the GB) offers trades of whatever goods they have for the needed goods. Since the trades are unbalanced in their favor - Iron Age for Modern Era, for example - the buyer also offers a FP donation to the 'seller' to make up for this difference. The donation goes to one or more of the seller's GBs. (Note: You'll need to be friends, guldmates, or neighbors in order to offer trades to one another.)
Things to work out beforehand
2) What GB(s) does the seller expect you to donate to? This will impact the rewards the buyer gets back.
3) What's the schedule for donations, and for trades? Does the buyer need to donate all the FPs first, then post the trades? Or trades first, then FPs over time? Or a fifth of the FPs, post one trade, more FPs, another trade, etc?
4) Finally, and probably most importantly, how many FPs total does the buyer need to donate? What is the cost of the GB kit?
It's not unheard of for the answers to 1-3 to impact #4. For example, a buyer offering iron for robots will probably pay top dollar (or top forge point), whereas one offering ferroconcrete for robots might get more favorable terms as the value of the traded goods is closer to fair. Similarly, a seller may want to use the forge points to level a low-age, low-reward building; but if the buyer wanted to get a higher value of rewards back, by donating to a high-age building, the seller wouldn't be out of line to ask for some additional FPs.
What's a fair price?
Good question! A couple of folks in Norasil are thinking about ways to gauge and report on the market for things like GB kits - if anyone in any world is doing the same, maybe report it here?
One last wrinkle to consider - it's not uncommon for their to be in-guild and out-of-guild prices for the same GB kit, with the in-guild price being lower.
Out of guild: 3:1 - 2.5:1 (so 625 -750 FPs for all 1,875 goods)