You can use Google Translate to communicate with your Francophone seller.
Insert the text you want to send in the English box. Make sure you have everything spelled correctly and that your grammar is correct too. The program will translate for you.
Then send the seller BOTH the English and the French message, noting that you used Google Translate.
You can pay for the item by going directly to your Paypal account and using Send Money. Note the eBay transaction number in the text box, both for your seller's bookkeeping and to show that you have really paid.