The flipping thing doesn't bother me. I think many of us do it at least a little.
I don't sell much, especially older stuff where I don't have extra copies of a card. But with newer stuff I have extras, sometimes loads of them.
Having sold at Shows, flea markets, ebay, here, and directly to dealers, overall, and I know this will be unusual, I prefer selling to dealers. I've never had it be a difficult process. I have these cards can you use any of them? Yes, this one, but I can only pay x for it. Ok, that's fine.
If there's negotiation, it's very brief, "I can pay 50" "How about 55?" " No, I can only do 50" or "sure that's not much more."
OR sorry, I can't go that low. (Never actually happened, probably because most direct single card sales are likely almost pre-sold so the offer is decent.)
I also enjoy some selling to collectors, but some make the process as drawn out and difficult as possible. "your card is horrible, but I really want/ need it because......so I'll pay you 10%" Umm....no thanks.
I will say that almost every sale here has been the better sort.
Steve B