I'm trying to get up the money. Is there some way you can hold onto this for me?
I'll even pay extra.
Since you like to make up and enforce the "rules" around here, you should probably already be aware that "I'll take it" is the accepted method of buying something here, not "can you hold it, till I don't know when, in case I may or may not ever get the money".
Play by the rules, or shop elsewhere. (I suggest Craigslist, seems to be something you're familiar with).