questions about buying/selling

I know these topics have been covered but the last, long threads have been about specific situations, with lots of emotion. I have a couple basic questions about buying and selling. I should say that I have no feedback on Audiogon or Audiomart; I have a modest number of positive reviews on eBay. 

These questions are in the context of Paypal reporting income and issuing a 1099k. If I'm understanding it correctly, any sales over $600 generate this taxable income requirement. It looks like many online sales now use the "family and friends" option.

BUYING: It looks to me like Audiomart offers no buyer protection. What protections are available to me as a buyer (other than research and due diligence)? If I insist on using PayPal goods/services (3%), does that provide me with adequate coverage through PayPal? I've read that using friends/family with a credit card offers coverage from the cc company, but I wonder if anyone out here has actually made a claim to a cc company this way. I wouldn't be surprised if a cc company was hesitant since family/friends is not supposed to be used for a purchase. Why would a cc company offer coverage on what should essentially be a cash transfer between friends/family? (And--there's a 3% charge to use a cc for family/friends, as I understand it.)

SELLING: I may want to sell a few things (CR-1, amp, etc). Without an established record, what assurances can I give a potential buyer here or on Audiomart? (I'm fine on eBay.) If I offer to use PayPal goods/services (and pay for it!), would that go far in reassuring a potential buyer that they're protected? 

Finally: I think I read that I wouldn't have to pay tax on income from sales if I can show that I am selling the items for less than I purchased them for. Is this true? And is it easy? (Sigh. I'll just end up paying my accountant to do all the filing work. There's no free lunch.)

I appreciate any general advice here and I'm sorry if I'm misunderstanding how PayPal works. If there's a *recent* thread on this topic that doesn't get into outraged specifics about individual situations, please redirect me. I'm only interested in how the policies work.