What do you call the "one who requests" and the "one who offers"?

In some sort of an exchange (maybe of services, maybe of items), there are those who seek and then request, and those who offer. What would be pleasent nouns to use for these roles? “requester”/”requestor” and “offerer” don’t sound right (“offerer” especially).

A shade of meaning that needs to be reflected in the nouns I’m interested in is that these cannot be ongoing services, i.e. the “offerer” does not offer you to also get some of what s/he is providing all the time to many people. If an agreement is made, the offerer will do something; otherwise s/he will not be doing it.

(Not a real) Example 1: Suppose it’s a forum in which people offer or request to have a stimulating intellectual conversation about some subject. Or offer to spot someone lifting weights and request someone to spot them. Offerers, and requesters.

I know there’s a related question on “someone who makes a request”, but I thought I’d ask about both terms together since the choice could be related.