Online selling etiquette

I go with whomever contacted me first, then work down the line if the first guy passes. Sometimes it’s frustrating. I may have to wait a couple of days for a guy to get back to me. Some ppl say they will take it then are not on line for a few days. That makes me think that they are not really interested. Just last weekend 2 guys wanted the same part. No one p/p me. I tell another guy on Thursday he can buy it. Well dam, I get up Friday morning and have pay pal from the first 2 guys. I refunded their money. 3rd guy got it as he paid right away. Kim