I'll answer to the best of my understanding. Paypal acts something as a conduit for funds from one person's bank to another person's bank. Some folks can set it up so that this conduit holds a portion of those funds, but that isn't really necessary as long as the buyer directs Paypal to pull the appropriate funds from his/her bank when he/she purchases something. Consequently, what you should look for is the finalized payment in your bank account. If you don't already know what your bank's policy regarding electronic funds transfer finalization, you should look into it just in case the buyer DOESN'T have enough funds in his/her bank account.
Can the buyer pull the funds back? This depends more on the merchant's site rather than Paypal's site. With E-Bay, for example, a buyer cannot pull the funds back (unless the buyer initiates a stop payment at his/her bank before the transfer was completed which would likely cost him/her a fee). The seller would have to initiate a refund. Check the rules and policies of the merchant's website to find out for certain.
Should you send the item before payment or after payment? That is a decision generally made by the seller. In my experience, the seller has always requested payment before sending out the merchandise. Is it a hugely expensive item? In such cases, sometimes merchants will accept half payment before shipping and half when the buyer receives the item. It all comes down to trust on the part of both buyer and seller.
Hope this helps,