You shouldn't post a form for something until you have the SCTs to pay for it.
Usually what people do, is they send a trade with whatever they're willing to offer for SCTs. If they're unsure, they send a trade suggestion, and I edit it back with how much SCTs they'd earn, or they just send a trade for the SCTs, and take what they get. I always let a person know how much SCTs they get when I accept.
Since it's a conversion, the person who wants the SCTs sends us the trade, that way they're only giving what they feel comfortable with.
The SCTs the person buys gets stored in the bank. Once they're there, they can post a form for whatever they want to spend it on ^^'