Yes, it is better to keep them separate.
Especially if the items you are buying are similar to the items you are selling.
It is best to use the high feedback account for your selling account. Which may mean ignoring the first bit of advice but consistency is the hobgoblin of small minds.
Your existing feedback won't make any difference to how your selling is treated by eBay, but will give your customers confidence in you.
Paypal will still be holding your customers' cleared payments against your good behaviour for 21 days before releasing them*.
You will still be restricted in the number and value of the items you offer for a few months while you find your feet.
* You will be able to print PP shipping labels from the Held funds, though.