Is it legal to do back-to-back visa runs?

Technically, visa runs are at the discretion of the border officer. If you have too many previous entries, you may be questioned or denied.

Can I use a credit card for the Cambodian visa?

No, you must pay in cash. USD is preferred, though they will accept Thai Baht at a poor exchange rate.

Performing a Visa Run from Thailand to Cambodia

To perform a visa run, take a bus or flight from Bangkok to the Poipet border crossing, exit Thailand, obtain a Cambodian visa, enter Cambodia, and immediately return through the border to re-enter Thailand. This process grants you a new 30-day or 60-day visa-exempt stamp, depending on your passport nationality.

Book your transport to the border. Take the early morning train from Hua Lamphong or Krung Thep Aphiwat station in Bangkok to Aranyaprathet (approx. 250 THB). From the station, take a tuk-tuk to the border gate for 100 THB.
Exit Thailand. Go to the Thai immigration office at the border. Submit your departure card and receive your exit stamp. Do not pay any 'fees' here; this is free.
Enter Cambodia. Walk to the Cambodian Visa-on-Arrival office. Pay the $30 USD visa fee. Ensure you have a passport-sized photo; if you don't, they will charge you an extra $2-5 'fine'.
Exit Cambodia. After getting your entry stamp, walk through the Cambodian immigration area. Turn around and head back to the exit office to stamp out of Cambodia immediately.
Re-enter Thailand. Present your passport to the Thai immigration officer. You will receive a new stamp allowing you to stay in Thailand for your allotted visa-exempt period.

Is it legal to do back-to-back visa runs?: Technically, visa runs are at the discretion of the border officer. If you have too many previous entries, you may be questioned or denied.
Can I use a credit card for the Cambodian visa?: No, you must pay in cash. USD is preferred, though they will accept Thai Baht at a poor exchange rate.