I assume you mean trade on the gray market, not melt.
The P-72 seems to go for about $45 at the moment but I see no reason why you wouldn't just take cash for it, even if you just put that back into RSI.
I haven't looked at the grey market in a long time, but RSI credit used to go for about 80% of face value. You buy a $10 RSI card for $8. It looks like the Archimedes goes for $50 so you could reasonably ask for that much cash or perhaps $65 in RSI credit.
LTI P-72's go for $40-$55 on the starcitizen trades reddit. You could just exchange it for credit but you'll only get $30 in credit, and I'd bet that there would be plenty of people who would buy it for $45 USD if you're willing to sell. Might as well take the extra cash even if you have to take a couple extra steps.
Or do as Mammon suggests and ask for $60+ in store credit.