LLance, I have a 200 rating on eBay with no negative or neutral feedbacks. I do take Paypal and I like it, although it will cost .50 or so to take credit cards. Ebay is moving to Paypal around the start of the year so you may as well get used to it.
If your grading skills are untested stick to Overstreet standards, but use vague grades like VG, F, VG/F. The 9.2, etc numeric system is tougher. Always slightly undergrade to kep you + feedback. Sell in smaller lots to attract buyers as shipping is always an added price and people get more for their money.
Hot books right now or recently have been the League of Extraordinary Gentlemen Mini..I got $85.00 for mine. Don't use vague titles..like 60's DC lot..no one will bid because the search won't register. Be exact as you can. I like 5-10 book lots. Be honest... I usually charge $4.50-5.00 for USPS Priority shipping..and try to sell 6 book lots because they fit in the flat rate envelope perfectly w/cardboard and you never get burned on shipping. Priority also speeds up the whole process.
Good luck... Oh yeah if you question a bidders integrity never leave feedback until the transaction is complete. I've heard of feedback blackmail...