I would treat them as two completely separate transactions. 

I would message the first purchase and apologise - hear me out. Message eg

" I'm terribly sorry to do this but the only way we can claim back our fees at this stage is to close the non-payment dispute as the echeque has not gone through. I'll wait for 48 hours to hear from you before closing the dispute. 

I'm more than happy to arrange collection if you can arrange payment within 48 hours however as a seller I cannot afford to pay selling fees to eBay on items that didn't actually sell. 

I hope your health improves soon and wish you well"

If they dare leave you bad feedback on the second, then you could apply to have it removed as it's about another transaction, but it might be tricky to proove it's because they're a non payer on another account? Do you have both accounts blocking buyers with strikes?