Would this be a good idea?

I would never recommend breeding to another animal unless they are quarantined for at least 3 months and preferably six. While I have done breeding loans with friends, I still quarantined. Since your female has been in contact with a male that's had an RI she could actually pose a risk for the other snake you plan to breed her to. The other guy should be making sure you quarantine her to be sure she's healthy. Since I'm not up on ball python genetics I'm not sure whether you'd even produce spiders or only het for spider if you breed your normal. Some genetics in ball morphs are co-dominant so you'll produce whether the one parent carries the gene or not. I'm also assuming that if he's attempting to sell the snake, he's not interested in keeping it around. Just loaning it out won't get his animal gone, if you get where I'm coming from. Personally, if it were me, I would not want to risk exposing my healthy, very expensive male to a possibly unhealthy female (and I would be certain you disclose that she was near or with a male that had an RI) that could pose a health risk. You never know though. I personally never accept breeding offers or loans except from people I know.

The spider is a male?I would check out the spider first. 300 is low for an adult spider even on craigslist so it may have a horrible wobble. But most likely if he wants it gone he isn't gnna allow u to use it for breeding unless u pay to own the snake. And since he is trying to get rid of it now it would take u a month or so to get him and the females cooled and locking up. Mist likely he will say no,but that's the worst he can say so asking wldnt hurt

Pet sales are prohibited on Craigslist. You already know he's either really, really, really stupid, too stupid to see all the warnings and prohibitions plastered all over the site saying not to sell pets on craigslist, or he's a cheating liar.

If he's a completely dishonest greedy jerk, and you want to go ahead and let him have access to your female, go for it...just be aware that your female is likely to "get sick and die" or otherwise vanish on you, so that he can double his profits. He's an unethical greedy jerk, right? He's already abusing a free website for profit. Why would you trust him at all?

Flag his ad, avoid people like that like you would avoid the plague. Want to breed your snake? Ask your vet if he knows or has a client that might be interested. Get on local herpetology boards and herp forums. Find someone that is honest and not a total idiot, and work with them. That is who you could trust with your snake's health and your potential profit. Not the Craigslist bozo.

Flag and avoid the craigslist idiot. Find someone decent to ask and possibly breed with.

The absolute best way to turn him on is to surprise him. Make the move on him in an unusual location or setting. Break any scheduling habits you may have also. The more you talk about it, he will probably grow uncomfortable. If you play the feminist point of view...He is not going to go for it.