Sorcha,Max has never had a problem accepting personal checks or money orders. If you click on the mudcat (fish) leaping out of the banjo, it will lead you to the snail mail addy. If I remember right it is 8 Gay Street, West Chester, PA, but don't remember the zip.
When I sold stuff at auction, the Mudcat and I have always been paid through the mail by personal check.
If you do have credit cards, you can call the toll free numbers at the Mudcat and Folk Legacy and I am sure Camsco (it is owned by Dick Greenhaus who is the God of the DT) and give them your info that way, rather than the over the net. I don't know if the others who share a percentage with the 'Cat have 800 numbers or not.
Hope this helps.
katlaughing