You may find that many sellers will not agree to use First Class mail as it is not trackable for paypal purposes.  If US sellers want paypal protection they must use International Priority or International Express which are fully trackable and insurable....and also very expensive!

Also, USPS will pick those services up from the seller whereas they have to go to the PO themselves if they use first class mail. If a seller is in a rural area they may live hours away from a PO so rely on their mail being picked up.