This was a long time ago, but I once negotiated a cash price for a car after doing research, getting information from that consumer reports car buying service, and visiting a few dealerships. They let me put $1000 with balance to be paid when I picked up the car. Then I thought, wow, instead of writing a check why not use a credit card, get all those frequent flyer miles, and just pay off the credit card in full when the card comes due. My credit limit wasn't high enough for this purchase, though, so I called my credit card company and they let me double my credit limit, so that my credit limit would be high enough to charge the car. Knowing retailers pay for credit card transactions, I brought both my check book and the credit card to the dealership when paying for and picking up the car, as I wasn't sure they would let me charge the car. What the dealership told me is that they couldn't give me the agreed upon cash price we had negotiated if I used an credit card and that they would need to charge we 3% more if I did that, so I just wrote a check.
Just saying you might have to pay more to do a purchase with a credit card or the seller might limit the dollars you can charge (as others have alluded to).
I often when doing a large purchase now will ask too if I can get a discount for cash. Usually the only time that this works for me and I get a discount is if I am working with the owner of a small business or something like that. Regular employees don't usually have authority to make deals with that in most businesses it seems.
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It seems pretty silly to me to have high credit limits that you don't use, but from what I've read for maximizing a credit score is that they like you to keep credit utilization under 30% of your credit limit. I don't really understand the thinking, though, as I would think it would be better to have a credit limit that is closer to the highest amount you would ever expect to have charged in a month vs. a limit that is way more than the maximum you ever expect you would charge (but what do I know and that's not how the calculations work). If you have a great credit score, though, a score dropping some points for a higher utilization limit doesn't matter, if your score is still in the range that is considered a great/good credit score. There is a big range that is considered to be a good credit score.