I do 300-400 a month these days, a reduction of almost half from a few years ago, and at least 20-30 are troublesome...a range of buyer scams and claims, from the comical to the outright scum 'want it for free'. There is no recourse for a business seller when a buyer options a return, accept it or get a defect, a few defects per sale ratio can drop your account into below average on the seller metrics, and at this point eBay 'hide' your listings and give you a couple of sales a week only. It can take months, years to restore an account to above average or TRS, it effectively kills businesses.
eBay does have the option now for Business Top Rated Sellers to reduce the refund amount by 50%. The buyer almost always appeals and eBay foots the bill for the remaining 50%.
Some of the wonderful lines you will receive from eBays 'support' staff when bringing up scams and abusive buyers is 'like a shop, you have to factor into loss through theft into your business plan', when it should really read, 'you have to factor into your business plan that eBay will always side with the buyer even with irrefutable evidence to show the scam, you are our customer, the seller, that pays our wages and the shareholders payouts are unimportant to us as there will always be someone else who till take your place'