I'm not a native Spanish speaker, but I did study Spanish in college. Here's my unsoliticed assistance in this matter:
"el" means "the" (as in "el gringo piense domaining significa vender dominios") or possibly "he" (as in "él no peude escribir en Español") but it's missing the accent, so probably not (have to have an IDN for that one). So if some unknown domainer actually for some reason adopted the English word "domaining" in their daily use, it would not begin with "el", but rather be simply "domaining", which of course is also the English word for "domaining". So, technically, the Spanish word for "domaining" in this case is "domaining". If you are selling domaining.com, then I will gladly make an offer.
As for "dominioing", again, we have a structural problem. Gerunds/past participles in Spanish do not end in "ing", but are verbs with an ending of "iendo" or "ando". Just like in English, we start with a verb, not a noun -- unless the verb also happens to be a noun, but that's just a coincidence. Domaining, in English, would be comprised of the verb domain, as in "to domain", a collocquial verb meaning roughly "to buy and sell domains". Dominio is a noun, so we need to convert it into a verb (why not -- I'm feeling a bit crazy). We should probably use "dominar", since this is the verb for "dominate" which etymologically is the closest word to "dominio" (also translated "dominion" -- you can see the relationship). In this case, dominar ends in "ar", so it would receive the "iando" ending = "dominiando" (also translated "dominating" -- could have another use for such a domain). If we leave it in it's original noun format and just turn it into a verb, I guess we could try using either suffix -- making it dominiando or dominiendo. Unforunately, dominiando.com is already taken (as most dictionary words are) -- and as luck would have it, it's already a domain registration site in Italy (Italian and Spanish are both romance languages, sharing many words, including this one). However, dominiendo.com is available for registration, if you like. Have at it my friend.
Hope this helps. Good luck.